Tumor Microenvironment : Extracellular Matrix Components – Part B [1st ed.] 9783030484569, 9783030484576

Table of contents :
Front Matter ....Pages i-xiii
Elastin in the Tumor Microenvironment (Yihong Wang, Elizabeth C. Song, Murray B. Resnick)....Pages 1-16
Decorin in the Tumor Microenvironment (Kornélia Baghy, Andrea Reszegi, Péter Tátrai, Ilona Kovalszky)....Pages 17-38
Syndecan-1 in the Tumor Microenvironment (Adriana Handra-Luca)....Pages 39-53
Versican in the Tumor Microenvironment (Athanasios Papadas, Fotis Asimakopoulos)....Pages 55-72
Chondroitin Sulphate Proteoglycans in the Tumour Microenvironment (Marta Mellai, Cristina Casalone, Cristiano Corona, Paola Crociara, Alessandra Favole, Paola Cassoni et al.)....Pages 73-92
Lipoproteins and the Tumor Microenvironment (Akpedje Serena Dossou, Nirupama Sabnis, Bhavani Nagarajan, Ezek Mathew, Rafal Fudala, Andras G. Lacko)....Pages 93-116
The Role of BEHAB/Brevican in the Tumor Microenvironment: Mediating Glioma Cell Invasion and Motility (Kristin A. Giamanco, Russell T. Matthews)....Pages 117-132
Thrombospondin in Tumor Microenvironment (Divya Ramchandani, Vivek Mittal)....Pages 133-147
Tenascin-C Function in Glioma: Immunomodulation and Beyond (Fatih Yalcin, Omar Dzaye, Shuli Xia)....Pages 149-172
Back Matter ....Pages 173-178

Citation preview

Advances in Experimental Medicine and Biology 1272

Alexander Birbrair  Editor

Tumor Microenvironment Extracellular Matrix Components – Part B

Advances in Experimental Medicine and Biology

Volume 1272 Series Editors Wim E. Crusio, Institut de Neurosciences Cognitives et Intégratives d’Aquitaine, CNRS and University of Bordeaux UMR 5287, Pessac Cedex, France Haidong Dong, Departments of Urology and Immunology, Mayo Clinic, Rochester, MN, USA Heinfried H. Radeke, Institute of Pharmacology & Toxicology, Clinic of the Goethe University Frankfurt Main, Frankfurt am Main, Hessen, Germany Nima Rezaei, Research Center for Immunodeficiencies, Children's Medical Center, Tehran University of Medical Sciences, Tehran, Iran

Advances in Experimental Medicine and Biology provides a platform for scientific contributions in the main disciplines of the biomedicine and the life sciences. This series publishes thematic volumes on contemporary research in the areas of microbiology, immunology, neurosciences, biochemistry, biomedical engineering, genetics, physiology, and cancer research. Covering emerging topics and techniques in basic and clinical science, it brings together clinicians and researchers from various fields. Advances in Experimental Medicine and Biology has been publishing exceptional works in the field for over 40 years, and is indexed in SCOPUS, Medline (PubMed), Journal Citation Reports/Science Edition, Science Citation Index Expanded (SciSearch, Web of Science), EMBASE, BIOSIS, Reaxys, EMBiology, the Chemical Abstracts Service (CAS), and Pathway Studio. 2019 Impact Factor: 2.450 5 year. Impact Factor: 2.324 More information about this series at http://www.springer.com/series/5584

Alexander Birbrair Editor

Tumor Microenvironment Extracellular Matrix Components – Part B

Editor Alexander Birbrair Department of Radiology Columbia University Medical Center New York, NY, USA Department of Pathology Federal University of Minas Gerais Belo Horizonte, Minas, Gerais, Brazil

ISSN 0065-2598     ISSN 2214-8019 (electronic) Advances in Experimental Medicine and Biology ISBN 978-3-030-48456-9    ISBN 978-3-030-48457-6 (eBook) https://doi.org/10.1007/978-3-030-48457-6 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

This book is dedicated to my mother, Marina Sobolevsky, of blessed memory, who passed away during the creation of this volume. Professor of Mathematics at the State University of Ceará (UECE), she was loved by her colleagues and students, whom she inspired by her unique manner of teaching. All success in my career and personal life I owe to her.

My beloved mom Marina Sobolevsky of blessed memory (July 28, 1959 – June 3, 2020).

Preface

This book’s initial title was “Tumor Microenvironment.” However, due to the current great interest in this topic, we were able to assemble more chapters than would fit in one book, covering tumor microenvironment biology from different perspectives. Therefore, the book was subdivided into several volumes. This book “Tumor Microenvironment: Extracellular Matrix Components – Part B” presents contributions by expert researchers and clinicians in the multidisciplinary areas of medical and biological research. The chapters provide timely detailed overviews of recent advances in the field. This book describes the major contributions of different extracellular matrix components in the tumor microenvironment during cancer development. Further insights into these mechanisms will have important implications for our understanding of cancer initiation, development, and progression. The authors focus on the modern methodologies and the leading-edge concepts in the field of cancer biology. In recent years, remarkable progress has been made in the identification and characterization of different components of the tumor microenvironment in several tissues using state-of-art techniques. These advantages facilitated identification of key targets and definition of the molecular basis of cancer progression within different organs. Thus, the present book is an attempt to describe the most recent developments in the area of tumor biology, which is one of the emergent hot topics in the field of molecular and cellular biology today. Here, we present a selected collection of detailed chapters on what we know so far about the extracellular matrix components in the tumor microenvironment in various tissues. Nine chapters written by experts in the field summarize the present knowledge about distinct extracellular matrix constituents during tumor development. Murray B. Resnick and colleagues from Brown University discuss Elastin in the tumor microenvironment. Kornélia Baghy and colleagues from Semmelweis University describe Decorin in the tumor microenvironment. Adriana Handra-Luca from Universite Paris Nord Sorbonne updates us with what we know about Syndecan-1 in the tumor microenvironment. Athanasios Papadas and Fotis Asimakopoulos from University of California San Diego address the importance of Versican in the tumor microenvironment. Marta Mellai and colleagues from Università del Piemonte Orientale compile our understanding of Chondroitin Sulphate Proteoglycans in the tumor microenvironment. Andras G.  Lacko and colleagues from The University of Texas Health Science Center at Fort Worth summarize current knowledge on vii

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Lipoproteins in the tumor microenvironment. Kristin A.  Giamanco and Russell T. Matthews from SUNY Upstate Medical University talk about the role of BEHAB/Brevican in the glioma microenvironment. Divya Ramchandani and Vivek Mittal from Weill Cornell Medicine focus on the effect of Thrombospondin in the tumor microenvironment. Finally, Shuli Xia and colleagues from Johns Hopkins School of Medicine give an overview of the Tenascin-C function in glioma. It is hoped that the articles published in this book will become a source of reference and inspiration for future research ideas. I would like to express my deep gratitude to my wife Veranika Ushakova and Mr. Murugesan Tamilsevan from Springer, who helped at every step of the execution of this project.

New York, NY, USA Alexander Birbrair Belo Horizonte, MG, Brazil

Preface

Contents

1 Elastin in the Tumor Microenvironment ��������������������������������������   1 Yihong Wang, Elizabeth C. Song, and Murray B. Resnick 2 Decorin in the Tumor Microenvironment��������������������������������������  17 Kornélia Baghy, Andrea Reszegi, Péter Tátrai, and Ilona Kovalszky 3 Syndecan-1 in the Tumor Microenvironment��������������������������������  39 Adriana Handra-Luca 4 Versican in the Tumor Microenvironment������������������������������������  55 Athanasios Papadas and Fotis Asimakopoulos 5 Chondroitin Sulphate Proteoglycans in the Tumour Microenvironment����������������������������������������������������������������������������  73 Marta Mellai, Cristina Casalone, Cristiano Corona, Paola Crociara, Alessandra Favole, Paola Cassoni, Davide Schiffer, and Renzo Boldorini 6 Lipoproteins and the Tumor Microenvironment��������������������������  93 Akpedje Serena Dossou, Nirupama Sabnis, Bhavani Nagarajan, Ezek Mathew, Rafal Fudala, and Andras G. Lacko 7 The Role of BEHAB/Brevican in the Tumor Microenvironment: Mediating Glioma Cell Invasion and Motility �������������������������������������������������������������� 117 Kristin A. Giamanco and Russell T. Matthews 8 Thrombospondin in Tumor Microenvironment���������������������������� 133 Divya Ramchandani and Vivek Mittal 9 Tenascin-C Function in Glioma: Immunomodulation and Beyond �������������������������������������������������������������������������������������� 149 Fatih Yalcin, Omar Dzaye, and Shuli Xia Index���������������������������������������������������������������������������������������������������������� 173

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Contributors

Fotis  Asimakopoulos Department of Medicine, Division of Blood and Marrow Transplantation, University of California San Diego (UCSD), Moores Cancer Center, La Jolla, CA, USA Kornélia  Baghy 1st Department of Pathology and Experimental Cancer Research, Semmelweis University, Budapest, Hungary Renzo Boldorini  Dipartimento di Scienze della Salute, Scuola di Medicina, Università del Piemonte Orientale (UPO), Novara, Italy Cristina  Casalone Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d’Aosta, Torino, Italy Paola Cassoni  Dipartimento di Scienze Mediche, Università di Torino/Città della Salute e della Scienza, Torino, Italy Cristiano  Corona Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d’Aosta, Torino, Italy Paola Crociara  Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d’Aosta, Torino, Italy Akpedje Serena Dossou  Lipoprotein Drug Delivery Research Laboratory, Department of Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, TX, USA Omar Dzaye  Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD, USA Department of Radiology and Neuroradiology, Charité, Berlin, Germany Alessandra  Favole Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d’Aosta, Torino, Italy Rafal Fudala  Lipoprotein Drug Delivery Research Laboratory, Department of Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, TX, USA Department of Microbiology, Immunology and Genetics, University of North Texas Health Science Center, Fort Worth, TX, USA Kristin  A.  Giamanco Department of Biological and Environmental Sciences, Western Connecticut State University, Danbury, CT, USA xi

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Adriana  Handra-Luca Service d’Anatomie pathologique; APHP GHU Avicenne, University Sorbonne Paris Nord, Bobigny, France Ilona  Kovalszky 1st Department of Pathology and Experimental Cancer Research, Semmelweis University, Budapest, Hungary Andras  G.  Lacko Lipoprotein Drug Delivery Research Laboratory, Department of Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, TX, USA Departments of Physiology/Anatomy and Pediatrics, University of North Texas Health Science Center, Fort Worth, TX, USA Ezek Mathew  Lipoprotein Drug Delivery Research Laboratory, Department of Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, TX, USA Russell T. Matthews  Department of Neuroscience and Physiology, SUNY Upstate Medical University, Syracuse, NY, USA Marta Mellai  Centro di Ricerca Traslazionale sulle Malattie Autoimmuni e Allergiche (CAAD), Università del Piemonte Orientale (UPO), Novara, Italy Dipartimento di Scienze della Salute, Scuola di Medicina, Università del Piemonte Orientale (UPO), Novara, Italy Fondazione Edo ed Elvo Tempia Valenta– ONLUS, Biella, Italy Vivek  Mittal Department of Cardiothoracic Surgery, Weill Cornell Medicine, New York, NY, USA Bhavani  Nagarajan Lipoprotein Drug Delivery Research Laboratory, Department of Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, TX, USA Athanasios  Papadas Department of Medicine, Division of Blood and Marrow Transplantation, University of California San Diego (UCSD), Moores Cancer Center, La Jolla, CA, USA University of Wisconsin-Madison, Cellular and Molecular Pathology Program, Madison, WI, USA Divya Ramchandani  Department of Cardiothoracic Surgery, Weill Cornell Medicine, New York, NY, USA Murray  B.  Resnick Department of Pathology and Laboratory Medicine, Rhode Island Hospital and Lifespan Medical Center, Warren Alpert Medical School of Brown University, Providence, RI, USA Andrea  Reszegi 1st Department of Pathology and Experimental Cancer Research, Semmelweis University, Budapest, Hungary Nirupama  Sabnis Lipoprotein Drug Delivery Research Laboratory, Department of Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, TX, USA Davide  Schiffer Ex Centro Ricerche/Fondazione Policlinico di Monza, Vercelli, Italy

Contributors

Contributors

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Elizabeth C. Song  Brown University, Providence, RI, USA Péter Tátrai  Solvo Biotechnology, Budapest, Hungary Yihong Wang  Department of Pathology and Laboratory Medicine, Rhode Island Hospital and Lifespan Medical Center, Warren Alpert Medical School of Brown University, Providence, RI, USA Shuli Xia  Hugo W. Moser Research Institute at Kennedy Krieger, Baltimore, MD, USA Department of Neurology, Johns Hopkins School of Medicine, Baltimore, MD, USA Fatih Yalcin  Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD, USA Department of Radiology and Neuroradiology, Charité, Berlin, Germany

1

Elastin in the Tumor Microenvironment Yihong Wang, Elizabeth C. Song, and Murray B. Resnick

Abstract

Elastic fibers are found in the extracellular matrix (ECM) of tissues requiring resilience and depend on elasticity. Elastin and its degradation products have multiple roles in the oncologic process. In many malignancies, the remodeled ECM expresses high levels of the elastin protein which may have either positive or negative effects on tumor growth. Elastin cross-linking with other ECM components and the enzymes governing this process all have effects on tumorigenesis. Elastases, and specifically neutrophil elastase, are key drivers of invasion and metastasis and therefore are important targets for inhibition. Elastin degradation leads to the generation of bioactive fragments and elastin-derived peptides that further modulate tumor growth and spread. Interestingly, elastin-like peptides (ELP) and elastin-derived peptides (EDP) may also be utilized as nano-carriers to combat tumor growth. EDPs drive tumor developY. Wang (*) · M. B. Resnick Department of Pathology and Laboratory Medicine, Rhode Island Hospital and Lifespan Medical Center, Warren Alpert Medical School of Brown University, Providence, RI, USA e-mail: [email protected] E. C. Song Brown University, Providence, RI, USA

ment in a variety of ways, and specifically targeting EDPs and their binding proteins are major objectives for ongoing and future anti-­ cancer therapies. Research on both the direct anti-cancer activity and the drug delivery capabilities of ELPs is another area likely to result in novel therapeutic agents in the near future. Keywords

Elastin · Elastic fiber · Elastin-binding protein · Elastosis · Elastoma · Extracellular matrix · Elastase · Neutrophil elastase · Elastin-like peptide · Elastin-derived peptide · Elastin receptor · Tumor-associated stroma · Lysyl oxidases · Elastin collagen cross-linking · Tumor microenvironment

1.1

Introduction

Elastin, one of the longest-lived proteins, is a major component of the extracellular matrix and confers flexibility to tissues requiring mechanical resilience. Elastin and its degradation products have multiple roles in the oncologic process. In many malignancies, the remodeled extracellular matrix (ECM) expresses high levels of the elastin protein which may have either positive or nega-

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 A. Birbrair (ed.), Tumor Microenvironment, Advances in Experimental Medicine and Biology 1272, https://doi.org/10.1007/978-3-030-48457-6_1

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Y. Wang et al.

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tive effects on tumor growth. Elastin cross-­ linking with other ECM components and the enzymes governing this process all have effects on tumorigenesis. During tumor progression, elastin in the ECM is enzymatically degraded allowing for local tumor spread and metastasis. Neutrophil elastase is one of the key and best studied elastases governing this process. Elastin degradation leads to the generation of bioactive fragments and elastin-derived peptides that further modulate tumor growth and spread. Interestingly, elastin-like peptides may also be utilized as nano-carriers to combat tumor growth. These topics will be expanded upon in the following chapters.

1.2

 lastin Fibers and Elastin E Synthesis

Elastic fibers are extracellular matrix components that endow connective tissue resilience and are present in large proportions in tissues requiring mechanical pliability such as the arteries, skin, lungs, and cartilage [1–3]. Elastic fibers are made up of two key components: (1) elastin, an amorphous protein that constitutes the fiber core, and (2) microfibrils made up of glycoproteins which surround the core [1, 4] (Fig. 1.1). The fundamental building block of elastin is the soluble monomeric protein tropoelastin. Humans have only one tropoelastin gene (ELN) [5]. The ELN gene is expressed during prenatal and postnatal life for the first few years. Its expression then drops to near-complete repression at maturity [6, 7]. Mature tropoelastin associates with the elastin-binding protein (EBP) intracellularly, and the tropoelastin-EBP complex is transported to the cell surface and secreted out of cells [8], where competition from extracellular galactosides results in the dissociation of tropoelastin from EBP; the latter is recycled back inside the cell [3] (Fig. 1.1). The ELN gene transcripts are subject to extensive alternative ­splicing which give rise to a variety of tropoelastin isoforms [9]. The tropoelastin released on the cell surface subsequently aggregates and deposits onto

microfibrils, composed of glycoproteins such as fibrillin-1 and fibrillin-2, microfibril-associated glycoprotein-1, EMILINs, latent transforming growth factor beta binding proteins, and others [10]. Microfibrils serve as a scaffold to direct tropoelastin alignment, cross-linking, and consequential elastic fiber formation [11]. Lysyl oxidase (LOX) deaminates lysine residues to form allysine, which reacts with adjacent allysine or lysine to form cross-links [12]. Further reaction can lead to two major amino acids in elastin  – desmosine and isodesmosine cross-links between tropoelastin molecules [13]. Cross-links within and between adjacent tropoelastin molecules result in the mature insoluble elastic fiber, which is a hydrophobic durable polymer, that is resistant to enzymatic proteolysis and experiences essentially no turnover in healthy tissues [14].

1.3

Tumor Elastosis

Stromal elastosis, defined as dense aggregates of elastic fibers, is a pathological stromal alteration seen in neoplastic tissue stroma. Tumor elastosis was first described in breast cancer by Cheatle and Cutler in 1931 [15], and the term elastosis was later coined by Jackson and Orr [16]. Elastosis has also been historically described as “scirrhous,” “chalky streaks” [17], and “amyloid” [18]. Elastosis is eosinophilic, pale, and homogeneous on the hematoxylin and eosin (H&E) stain and can be distinctively highlighted on elastic-van Gieson’s (EVG) stain and immunohistochemically using anti-elastin antibodies (Fig. 1.2).

1.3.1 Elastosis in Breast Cancer Breast cancer has been the most comprehensively studied of all malignancies exhibiting elastosis. The first detailed morphological description of elastosis in breast cancer was provided by Lundmark and by Shivas et al. in 1972 [19, 20]. In 1974, Azzopardi and Laurini described two types of elastosis occurring in breast cancer: peri-

1  Elastin in the Tumor Microenvironment

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Fig. 1.1  Elastin biosynthesis. Tropoelastin is transcribed and translated from the elastin (ELN) gene and transported to the plasma membrane in association with elastin-­binding protein (EBP) to prevent its aggregation and premature degradation. EBP-tropoelastin assembly is then directed to the plasma membrane. EBP is secreted, disassociates from the complex, and binds to galactose

sugars and recycles back into the cell nucleus. Tropoelastin is released and aggregates on the cell surface. Tropoelastin aggregates are oxidized by lysyl oxidase leading to cross-­ linked elastin that accumulates on microfibrils which help to direct elastin deposition. The process of deposition and cross-linking continues to give rise to mature elastic fibers

ductal and vascular. They described elastosis occurring in 90% of infiltrating breast carcinomas and postulated that breast cancer cells secreted factors which induced fibroblasts to produce elastin [21]. They further suggested that elastosis may be an early indicator of tumor invasion. This concept was elaborated on by Lundmark who maintained that elastosis accompanying ductal carcinoma in situ is an indicator of early invasion and correlates with age, tumor type, and grade [19]. The degree of periductal elastosis and stromal elastosis increases progressively with the severity of breast disease [22, 23],

and several studies have correlated elastin with estrogen receptor content [24–28]. The prognostic significance of elastosis in breast cancer is unclear [20, 25, 29]. Rasmussen et al. failed to demonstrate any prognostic significance of elastosis in a group of 171 primary breast carcinomas [30]. In a more recent study, Chen et al. established that the presence of elastosis was associated with low tumor cell proliferation and a good prognosis [31]. Masters et al. found a positive correlation between tumor elastosis and endocrine therapy response [32]. It has been suggested that the source of elastic fibers in breast cancer may result from neoplastic

Y. Wang et al.

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Fig. 1.2  Cancer-associated stromal response with prominent elastosis. (a and b) Breast cancer H&E and elastic stain; (c and d) lung cancer H&E and elastic stain. (Magnification 400×)

epithelial cells provoking stromal fibroblasts [21, 23, 33, 34] or from the neoplastic epithelial cells themselves [20, 35]. In a largely immunohistochemical study, Mera et  al. identified elastic fibers in breast cancer stroma [36]. Using mRNA in situ hybridization, Krishnan and Cleary demonstrated that in some invasive breast cancers, stromal cells are the source of elastotic materials, whereas in others the malignant epithelial cells are responsible [37]. Breast cancer cell line stud-

ies further confirmed that tumor cells are capable of producing tropoelastin [37, 38].

1.3.2 E  lastosis in Other Solid Tumors Elastosis has been also been described in tumor-­ associated stroma of the lung, salivary gland, thyroid, cervix, stomach, and prostate [39–42].

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degradation can modulate the behavior of a broad range of biological activities including chemotaxis, proliferation, and protease release. In cancers, elastin peptides have been shown to modulate the cellular physiology of tumor cells, stromal fibroblasts, smooth muscle cell, endothelial cells, and inflammatory cells [51–54]. The chemotactic effect of elastin peptides has been mainly studied in monocytes. The κ-elastin induces an increase of intracellular cyclic 3,5_-adenosine monophosphate (cAMP), cyclic 3,5_-guanosine monophosphate (cGMP), and Ca2+ levels and stimulates the respiratory burst in monocytes; the authors suggested the increased cGMP level related to chemotaxis. The effect of elastin peptides on intracellular calcium level and cGMP levels plays a role in the modifications of the extracellular matrix following elastin degradation as observed in atherosclerosis [55, 56]. A study by Pocza et al. found that elastic peptides have a potent chemotactic effect on melanoma cells and their presence at a distant organ might 1.4 Elastin-Derived Peptides contribute to metastasis [57]. Recently, elastin peptides have been linked to in vivo regulation of The functional form of elastin is a highly cross-­ tumor progression of melanoma, which also has linked polymer that organizes as sheets or fibers an elastin-rich tumor-associated stroma [58]. in the extracellular matrix. The fact that purified The signaling pathways related to elastin elastin remains insoluble [46] severely limits its peptide-­induced proliferation have been recently use in biological experiments. As a consequence, studied in porcine arterial smooth muscle cells elastin-derived peptides (EDP) have been widely [59]. The authors have shown that elastin hydroused, allowing considerable advances in the lysates and synthetic peptides trigger signaling understanding of elastin fiber aging and remod- pathways leading to the opening of L-type caleling. The term “elastin peptides” designates cium channels and activation of pertussis toxin-­ both enzymatically and chemically produced sensitive G proteins. Moreover, they point out peptides. The former are the results of elastin that c-Src, the Ras/Raf/MEK/ERK signaling casdigestion by elastases. They are often termed cade, and the platelet-derived growth factor elastin lysate. The latter corresponds either to receptor are also involved in elastin peptide-­ synthetic peptides or to the peptide mixture induced proliferation. EDPs induced in vitro proobtained after mild chemical hydrolysis of insol- liferation have been shown in several neoplastic uble elastin by oxalic acid [47]. Elastin is highly cell lines including glioma, astrocytoma, and resistant to proteolysis and experiences essen- melanoma [60]. Surface aggregation of elastin tially no turnover in normal physiological condi- receptor molecules caused by suramin amplified tions. However, members of the serine, aspartate, signals leads to proliferation of human glioma and cysteine proteases and matrix metallopro- cells. Further evidence for EDP-induced prolifteinases (MMP) superfamilies can fragment erative potential can be seen in tropoelastin and elastin to elastin peptides during several patho- EDPs, which promote proliferation of human logical and physiopathological processes [48– astrocytoma cell lines [61, 62] are additional evi50]. The elastin peptides generated during elastin dences of EDPs induced proliferative potential. Elastosis in lung carcinomas is more commonly seen in adenocarcinomas than squamous cell carcinomas and was not detected in small-cell or large-cell carcinomas [43]. Elastosis in pulmonary adenocarcinomas was more commonly associated with well-differentiated tumors and was associated with better patient outcome in low-stage tumors [43]. Azzopardi and Zayid first described the association of elastosis with salivary gland tumors [44], and David et al. described the presence of elastosis in pleomorphic adenomas, malignant pleomorphic adenomas, and adenoid cystic neoplasms of the salivary glands but not in other variants of salivary gland tumors [45]. Elastosis is identified more frequently with classical papillary thyroid carcinomas than the follicular variant. In papillary carcinomas, deposits of elastic fibers vary in size and shape and are more frequently distributed at the periphery of the tumor tissue [41].

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The synthesis of proteases such as matrix metalloproteinases (MMP) by tumor and stroma cells during stromal reaction is one of the crucial events leading to matrix degradation and tumor progression [63]. Elastin peptides upregulate pro-MMP-1 production in skin fibroblasts and HT-1080 tumor cells [64, 65]. EDPs enhance melanoma growth in vivo by upregulating the activation of Mcol-A(MMP-1) collagenase [62]. In addition, Toupance et  al. demonstrated that elastin peptides stimulate lung cancer cell invasion post-transcriptionally by regulation of MMP-2 and urokinase plasminogen activator [66].

1.4.1 Elastin-Derived Peptides and the Elastin Receptor Elastin-derived peptides display a wide range of biological activity by interacting with the elastin receptor complex (ERC). The ERC is expressed on the surface of numerous cell types [67]. Three cell surface receptors have been described to mediate the effects of elastin peptides: ERC, aVb3 integrin, and galectin-3. ERC, the primary receptor, is comprised of three subunits: the elastin-­binding protein (EBP), neuraminidase-1 (Neu-1), and the protective protein/cathepsin A (PPCA) [68–70]. The 67  kDa elastin-binding protein (EBP) is peripheral and binds the VGVAPG domain on tropoelastin with high affinity. It also possesses galactolectin properties and therefore has the ability to fix β-galactosugars such as galactose or lactose. EBP, identified as an enzymatically inactive form of β-galactosidase, is the subunit which actually binds the elastic peptide [71]. The two other subunits are membrane-associated, and the catalytic activity of Neu-1 is crucial for elastin receptor complex signaling [72]. Typical elastin receptor complex ligands are peptides containing the xGxxPG (where x represents any amino acid) consensus sequence thought to favor a local type VIII b-turn conformation [73]. This conformation has been linked to the bioactivity of the VGVAPG peptide [74, 75].

Y. Wang et al.

αVβ3 integrin is another cell surface receptor mediating elastin peptide signaling. Traditionally, integrins bind to extracellular matrix through RGD motifs [76]. However, there are several non-RGD sequences serving as ligands for integrins [77]. Recently, the aVb3 integrin has been reported to interact directly with tropoelastin and the elastin peptide to mediate their effects [78, 79]. The third elastin peptide receptor is galectin­3. Galectin-3 is a 31 kDa β-galactoside-binding lectin and plays an important role in cell-­ extracellular matrix interactions [80]. Its expression has been linked to tumor progression and cancer aggressiveness [81]. Galectin-3 is capable of regulating the interactions between cells and elastin and to bind the VGVAPG elastin peptide leading to melanoma invasion [79, 82].

1.4.2 Elastin-Like Polypeptides and Cancer Therapy Identifying the nature of elastin-derived proteins’ cryptic sequences and the characterization of their bioactive structures may be utilized for the conception of antagonists blocking the elastin-­ binding site. Elastin-like polypeptides (ELPs) are comprised of a genetically engineered class of molecules derived from tropoelastin. Such substances could have anti-cancer properties such as reducing cell proliferation, chemotactic response of malignant cells, and MMP synthesis as reviewed recently by Despanie et  al. [83]. In addition to these cancer treatment applications, ELPs may represent a promising class of recombinant biopolymers for the delivery of drugs and imaging agents to solid tumors via systemic or local administration. Small hydrophobic drugs can be conjugated to the C-terminus of the elastin-­like polypeptides to impart the amphiphilicity needed to mediate the self-assembly of nanoparticles [84]. These systemically delivered ELPs –drug nanoparticles – preferentially localize to the tumor site via the permeability and retention effect, resulting in reduced toxicity and enhanced treatment efficacy [85]. In order to improve the pharmacokinetic profiles of the ELP drug delivery platform, researchers have modi-

1  Elastin in the Tumor Microenvironment

fied the system using genetically engineered ELP incorporated with multiple copies of the IL-4 receptor targeting peptide (AP1) and the proapoptotic peptide (KLAKLAK)2 referred to as AP1-ELP-KLAK. Systemic administration of AP1-ELP-KLAK significantly inhibited tumor growth by provoking cell apoptosis in various tumor xenograft models without any specific organ toxicity and also improved drug bioavailability, stability, membrane penetration, and drug half-life [85]. A newer application using a bladder tumor-targeting peptide – embedding ELP as a drug delivery vehicle  – displayed excellent localization in bladder tumor-xenografted mice after intravenous injection and was strictly confined to specific antigen-overexpressing tumor tissue [86]. Other approaches to use elastin-like polypeptides in anti-cancer drug delivery take direct advantage of the thermal responsiveness of elastin-­like polypeptides. At physiological temperatures, ELPs are entirely soluble, but at higher temperatures, they become insoluble by coacervation [87]. ELP-blocking copolymers may be designed to assemble into nanoparticles in response to hyperthermia due to the independent thermal transition of the hydrophobic block, thus resulting in multivalent ligand display of a ligand for spatially enhanced vascular targeting. Delivery of ELPs conjugated with radiotherapeutics maybe injected directly into tumor where they undergo coacervation to form an injectable drug depot for intratumoral delivery. These injectable coacervate ELP-radionuclide depots display a long residence in the tumor and result in inhibition of tumor growth [88].

1.5

Elastase and the Degradation of Elastin in Tumorigenesis

During invasion and metastasis, tumor cells confront a variety of natural tissue barriers in vivo, such as basement membranes and surrounding tissue stromal matrices including elastin. It is thus necessary for tumor cells to elaborate a battery of extracellular matrix degradative enzymes

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to achieve invasion and metastasis. Many different types of extracellular matrix degradative enzymes have been implicated in the invasive growth and metastasis of cancer cells [89–91]. Elastases are a heterogeneous group of enzymes capable of degrading mature, insoluble elastin protein under physiological conditions. They are found in most of the major proteolytic families, including serine, thiol, aspartic enzymes, and metalloenzymes [92]. There are three well-­ characterized mammalian elastases: (1) pancreatic elastase I, a serine protease secreted in zymogen form by pancreatic acinar cells; (2) neutrophil elastase, a neutral protease found in granules of human polymorphonuclear leukocytes [93, 94]; and (3) metalloprotease which is secreted by macrophages. Four metalloproteases (MMP-2, MMP-7, MMP-9, and MMP-12) are elastases [95]. Neutrophil elastase that belongs to the serine proteinase enzymes family exhibits the most potent proteolytic activity under physiological conditions and is also the most widely studied elastase. Neutrophil elastase was first described as a serine protease stored in azurophilic granules of neutrophils. It is released into the extracellular space through degranulation or during neutrophil extracellular trap formation to carry out its physiological function of pathogen clearance during infection. Neutrophil elastase is also a critical regulator of the inflammatory response [96, 97]. Neutrophil elastase is implicated in matrix remodeling in a variety of pathological processes including chronic obstructive pulmonary disease [98], pulmonary fibrosis [99], and atherosclerosis [100]. During lung cancer progression, extensive destruction of the rich elastin network through the expression and activation of elastases is observed [101]. Elastinolytic activities in human breast cancer tissue have been demonstrated by Hornebeck et al. [102]. Thereafter, several investigators have described elastinolytic enzyme production by human and rodent mammary tumor cells [103–105]. In lung cancer patients, elevated serological neutrophil elastase positively correlates not only with disease state but also with disease progression [106]. Neutrophil elastase activity is three- and fivefold greater in the bron-

Y. Wang et al.

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choalveolar lavage fluid and serum, respectively, in individuals with lung cancer compared to those with COPD [107]. Enhanced neutrophil elastase activity in lung cancer patients can also be detected indirectly through accumulation of neutrophil elastase-specific elastin degradation products [108]. A strong neutrophil elastase proteolytic fingerprint distinguishes the colon adenocarcinoma proteome from that of ulcerative colitis [109]. In breast cancer, high neutrophil elastase immunoreactivity is an independent poor prognosis factor correlated with diminished metastasis-free survival, relapse-free survival, and overall survival [110–112]. Neutrophil elastase is an integral component of neutrophil extracellular traps (NETs). Myeloid cells secrete neutrophil elastase and neutrophil extracellular traps (NETs) in response to cues within the tumor microenvironment, thereby leading to enhanced activity in cancer cells [113– 115]. A recent study showed that DNA released from NETs activates pancreatic stellate cells and enhances pancreatic tumor growth [116]. Neutrophil elastase is likely a key driver of tumorigenesis and facilitator of metastasis, since genetic deletion and pharmacological inhibition markedly reduce tumor burden and metastatic potential in some studies [117–119]. Neutrophil elastase may therefore serve as a novel cancer biomarker or therapeutic target.

1.6

Elastin Collagen Cross-Linking

In the tumor microenvironment, fibrillar collagens represent the most abundant extracellular matrix proteins. Formation of covalent c­ ross-­links occurring between elastin and fibrillar collagen promotes the tensile strength of the ECM and is mediated by the action of lysyl oxidase (LOX) [120–123].

1.6.1 L  ysyl Oxidases Role in Elastin Collagen Cross-Linking While intracellular functions have been reported for LOX proteins, the primary role of this family of enzymes is the remodeling of the extracellular matrix. The best-studied role of LOX in the extracellular matrix is the cross-linking of collagens and elastin. The LOX family constitutes five members of extracellular copper-dependent amine oxidases including LOX and LOX-like isoforms (LOXL) 1–4 which are present in the extracellular matrix [reviews seen 124–125]. Collagen is the most abundant, naturally occurring protein in mammals. It is found in the bone, teeth, skin, ligaments, and tendons. Of the 27 naturally occurring collagen types, type I collagen is the most abundant type. Type I collagen is made up of two identical α1(I) chains and one α2(II) chain. These chains have a common repeating motif, Gly e XeY, where X and Y are primarily proline and hydroxyproline residues [126]. LOX catalyzes a key step in the cross-­ linking of collagen and elastin where lysine residues within the N- and C-terminal telopeptide regions are oxidized to aminoadipic d-­semialdehyde [127]. These resulting aldehydes are condensed with unmodified lysine and hydroxylysine residues, creating cross-linkages. Examples of the cross-linking reactions are given in Fig.  1.3 [adapted from 124]. Elastin is also modified by LOX; these properties are important to the extracellular matrix because elastin must be able to adapt to its environment while at the same time retains its resilience. The mechanism for the formation of elastin cross-links is very similar to that of collagen, except that the cross-­ linking in elastin does not involve hydroxylysine, whereas desmosine and isodesmosine are not present in collagen [128]. Levental and colleagues showed that by modifying the state of collagen cross-linking and ECM stiffness, two physical parameters of the tissue microenvironment, the invasive behavior of an oncogene pre-­ transformed mammary epithelium could be modulated. This was characterized by promotion of focal adhesions, enhanced PI3 kinase (PI3K) activity, and induced invasion of an oncogene-­

1  Elastin in the Tumor Microenvironment

Fig. 1.3  Lysyl oxidase initiated cross-link formation in tropocollagen and tropoelastin. Lysyl oxidase catalyzes the oxidative deamination of lysine and hydroxylysine residues in tropocollagen and tropoelastin (only lysine residues are shown). The product allysine residues spontaneously react with other allysine or lysine residues via

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aldol condensation or Schiff base formation. The bifunctional condensation products can further cross-link to form tri-, tetra-, or even pentafunctional cross-links (depicted is desmosine, a common tetrafunctional cross-­ link found in elastin)

initiated epithelium, even in the absence of cel- scope of this chapter and was recently reviewed lular and soluble tissue and systemic factors. The by Johnson and Lopez [125]. findings imply that changes of extracellular matrix-induced tissue fibrosis could regulate cancer behavior by influencing the biophysical prop- 1.6.2 Collagen Elastin Cross-Linking in Tumorigenesis erties of the microenvironment to alter force at the cell and/or tissue level [129]. LOX enzymes are widely expressed in tumor-­ Collagen type IX α-1 (ColXα1) is a short-chain associated extracellular matrix and possess a collagen, typically found underlying endothelial wide range of biological functions other than col- cells and in the hypertrophic zone of cartilage lagen/elastin cross-linking. During tumor devel- during endochondral ossification where it particiopment, tumor cells constantly communicate pates in calcifying cartilage formation [12]. We with the surrounding microenvironment to sup- have recently shown that increased ColXα1 preport tumor cell proliferation, epithelial-to-­ dicts poor pathologic response in neoadjuvant-­ mesenchymal transition (EMT), migration, treated ER+/HER2+ breast tumors [130]. invasion, angiogenesis, and metastasis. Aberrant Interestingly we also observed that ColXα1 expression or activation of LOX alters the cellu- expression in breast tumors has a patchy distribular microenvironment, leading to many diseases tion pattern reminiscent of elastosis. We further including atherosclerosis, tissue fibrosis, and demonstrated using immunohistochemistry, cancer. This topic is somewhat removed from the immunofluorescence, and electron microscopy

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Fig. 1.4 (a) Elastin and collagen expression in normal breast tissue, ductal carcinoma in situ (DCIS), and invasive ductal carcinoma. Elastin is focally expressed in normal breast stroma, but not ColXα1 (first row). In DCIS, elastin and collagen expression were both present in a periductal pattern and within the stroma (second row). In invasive carcinoma, elastin and collagen were co-­ expressed in tumor-associated stroma (third row). (b) Immunofluorescence staining of elastin and ColXα1  in tumor-associated stroma. DAPI (blue) highlights the tumor; elastin (green) and ColXα1 (red) are distributed and co-localized in tumor-associated stroma (yellow). The tumor and tumor stroma are illustrated with the merge of

Y. Wang et al.

DAPI (tumor) and elastin (tumor stroma). (c) Immune electron microscopy of ColXα1 and elastin localization in breast tumor stroma using double labeling. The micrograph shows patchy amorphous material in the extracellular space near a fibroblast nucleus which is double stained with gold-conjugated anti-ColXα1 antibody (10  nm gold particles) and gold-conjugated anti-elastin antibody (25  nm gold particles). The field also contains collagen fibrils (C), collagen elastin complex (CE), and fine cytoplasmic extension of the fibroblasts (F). Original magnification (X25000). Larger particles, elastin (arrow); smaller particles, ColXα1 (in arrow head)

1  Elastin in the Tumor Microenvironment

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Fig. 1.4 (continued)

that ColXα1 co-localizes with elastin in invasive breast cancer-associated stroma [131] (Fig. 1.4), suggesting that this co-localization is specific for neoplastic transformation. Using radioactive labeling and HPLC methodology, Kao et  al. found that the synthesis of collagen and elastin increased by 50% and 70%, respectively, in desmoplastic breast cancer stroma on a per-cell basis, further suggesting that interplay between these two proteins may be critical in the neoplastic process [132].

1.7

Future Directions

Surprisingly, despite observations dating back to more than a century, little is known of how intact elastin directly effects tumorigenesis, and even less is known of the nature of other macromolecules that interact and cross-link with elastin which also play a role in neoplasia. Newer technologies targeting the stromal-epithelial interface such as organoid culture and single cell expression analysis will likely shed light on these interactions. Elastases, and specifically neutrophil elastase, are key drivers of invasion and metastasis and are therefore an important target for inhibition. As

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neutrophil elastase is an integral component of neutrophil extracellular traps (NETs) and NETs modulate tumor growth, NETs are also potential therapeutic targets. As stated previously in this review, due to the insoluble nature of elastin, much of the research governing elastin biology in tumorigenesis is related to EDPs and ELPs. EDPs drive tumor development in a variety of ways, and specifically targeting EDPs and their binding proteins are major objectives for ongoing and future anti-­ cancer therapies. Research on both the direct anti-cancer activity and the drug delivery capabilities of ELPs is another area likely to result in novel therapeutic agents in the near future.

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2

Decorin in the Tumor Microenvironment Kornélia Baghy, Andrea Reszegi, Péter Tátrai, and Ilona Kovalszky

Abstract

The tumor microenvironment plays a determining role in cancer development through a plethora of interactions between the extracellular matrix and tumor cells. Decorin is a prototype member of the SLRP family found in a variety of tissues and is expressed in the stroma of various forms of cancer. Decorin has gained recognition for its essential roles in inflammation, fibrotic disorders, and cancer, and due to its antitumor properties, it has been proposed to act as a “guardian from the matrix.” Initially identified as a natural inhibitor of transforming growth factor-β, soluble decorin is emerging as a pan-RTK inhibitor targeting a multitude of RTKs, including EGFR, Met, IGF-IR, VEGFR2, and PDGFR. Besides initiating signaling, decorin/ RTK interaction can induce caveosomal internalization and receptor degradation. Decorin also triggers cell cycle arrest and apoptosis

K. Baghy (*) · A. Reszegi · I. Kovalszky 1st Department of Pathology and Experimental Cancer Research, Semmelweis University, Budapest, Hungary e-mail: [email protected]; [email protected] P. Tátrai Solvo Biotechnology, Budapest, Hungary e-mail: [email protected]

and evokes antimetastatic and antiangiogenic processes. In addition, as a novel regulatory mechanism, decorin was shown to induce conserved catabolic processes, such as endothelial cell autophagy and tumor cell mitophagy. Therefore, decorin is a promising candidate for combatting cancer, especially the cancer types heavily dependent on RTK signaling. Keywords

Decorin · Extracellular matrix · Receptor tyrosine kinase · Autophagy · Mitophagy · Inflammation · SLRP · Tumor · Stroma · EGFR · Met · Angiogenesis · Cell cycle · Signaling · Growth factor

2.1

Introduction

Cancer is a rising pandemic and a major public health concern worldwide, with about 18 million new cases and over 19 million cancer deaths each year [1, 2]. Understanding the pathophysiology and molecular mechanisms of tumorigenesis is fundamental to the development of new therapeutic agents and methods against the disease. Neoplastic growth has long been viewed as a result of activating mutations in oncogenes and silencing of tumor suppressor genes in tumor

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 A. Birbrair (ed.), Tumor Microenvironment, Advances in Experimental Medicine and Biology 1272, https://doi.org/10.1007/978-3-030-48457-6_2

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cells that collectively provide selective advantage fibrillogenesis; its appearance as “decoration” on for proliferation, survival, and migration. collagen fibrils led to the eponym decorin [10– However, the discovery of a myriad changes in 14]. The DCN gene was cloned in 1986, and the the tumor microenvironment that is known to protein was initially thought to be only a struccoordinate many aspects of tumorigenesis has tural constituent of the ECM [15]. Soon, howshifted this paradigm. The tumor stroma is now ever, it was discovered that decorin was able to considered as an active participant that plays affect multiple cellular functions such as prolifdetermining roles by enhancing growth promot- eration, differentiation, migration, and spreading, ing signals, downregulating apoptotic mecha- as well as inflammatory responses [16–19]. nisms, and facilitating neoangiogenesis [3]. These early studies conducted on tumor cells Consequently, the last decade has seen a surge of were the first reports on the tumor-suppressive interest in the role of the stroma in carcinogene- effects of decorin and initiated the efforts, still sis, and research is now focusing not only on ongoing, to leverage the anticancer properties of tumor cells but also on the surrounding tumor decorin for tumor therapy [20]. stroma, including the abnormal synthesis and This chapter will highlight the cancer-related deposition of proteoglycans. aspects of the cellular and molecular roles of The extracellular matrix (ECM) is a complex, decorin. We will first describe its distribution in well-organized structure of macromolecules that cancer. Next, the structural basis of its antitumor interact with each other and with the resident properties will be discussed, followed by a cells of the tissue. Matrix macromolecules pro- detailed examination of the classical and novel vide structural integrity and influence cell growth, signaling mechanisms of decorin-mediated oncomigration, and differentiation. During the pro- suppression. Finally, we will provide an overview cess of tumorigenesis, the ECM undergoes quan- of studies aiming at the therapeutic application of titative and qualitative changes. The tumor stroma decorin against cancers. consists of ECM components such as proteoglycans (PGs), collagens, fibronectin, laminins, The Emergence of Decorin hyaluronic acid (HA), (glyco)proteins, as well as 2.2 growth factors, chemokines, and cytokines stored as an Oncosuppressive within the ECM. Beside noncellular components, Molecule various cell populations such as immune cells, fibroblasts, endothelial cells reside in the matrix, 2.2.1 Misexpression and Localization of Decorin and together with tumor cells, they are responsiin Cancer ble for ECM production [4, 5]. Tissue remodeling is a characteristic feature of tumor development, whereby changes in the number To understand the biological role of decorin in and types of resident cells occur together with the cancers, its expression patterns and localization radical transformation of ECM structure and within tumors need to be discussed first. function [6]. The remodeled ECM of the tumor Compared to the vast number of studies in vitro, stroma, enriched in proteoglycans, typically sup- the expression of decorin in tumorous tissues is ports the cancerous phenotype and promotes can- relatively unexplored. Nevertheless, existing cer cell aggressiveness [5, 6]. The diverse reports on decorin expression in various tumors regulatory features ECM molecules can display of different grade and origin reveal a general tenare well exemplified by the family of small dency of downregulation in the parenchyma of leucine-­rich proteoglycans (SLRP) [7–9]. advanced tumors, occasionally counterpointed Decorin is the prototypic and best-­by marked overexpression in the stroma. In characterized member of the SLRP family. human breast carcinoma, decorin is downreguDecorin was originally discovered as a strong lated both at the mRNA and protein level combinding partner of collagen necessary for proper pared to normal tissues as well as to non-tumorous

2  Decorin in the Tumor Microenvironment

adjacent tissues of cancer patients [21]. Similar tendencies of decorin downregulation were observed in several other malignancies, such as lung [22], ovarian [23], and endometrial cancers [24]. In addition, in node-negative invasive breast carcinoma and soft tissue tumors, reduced expression of decorin is associated with poor prognosis [25, 26]. According to the Human Protein Atlas database, significant reduction in decorin levels was found in the stroma of several tumor types such as the breast, cervical, bladder, colon, kidney, pancreas, ovary, prostate, skin, stomach, rectal, and testis [9, 27]. Decorin expression was diminished in the stroma of lowand high-grade urothelial carcinoma, while high levels of decorin were seen in the submucosa and deep tumor stroma [28]. Decorin expression was found to be decreased in multiple myeloma and monoclonal gammopathy of undetermined significance [29], in esophageal squamous cell carcinoma [30], and in some cases of colon carcinoma [31]. Within the tumor parenchyma, several studies reported the complete absence of decorin expression in various tumors including urothelial, prostate, myeloma, and liver cancer [32–37]. While tumor cells and non-tumorous connective tissues usually contain no or little decorin, the tumorous stroma may produce large amounts of this proteoglycan as seen in human colon [38– 40] and breast carcinomas [41, 42]. In these cases it is conceivable that despite its loss from tumor cells, the stromal accumulation of decorin still impedes tumor growth by forming a physical barrier as seen in desmoplastic reactions. In addition, large amounts of decorin in the ECM of stromal origin may inhibit receptor tyrosine kinases (RTKs, see details later) on tumor cell membranes in a paracrine fashion. Along with other proteoglycans, decorin expression is also highly upregulated in the ECM of pancreatic carcinoma, typically seen at the border of tumorous areas, and pancreatic stellate cells actively overproduce decorin [20, 43, 44], whereas pancreatic cancer cells exhibit a complete loss of decorin. Intriguingly, transcriptional analysis of tumor progression at the mRNA level revealed high decorin expression during the early stages of

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tumorigenesis. In B-cell chronic lymphoid leukemia (CLL), high decorin mRNA levels were detected in early stages in contrast to suppression in advanced stages. In line with this, patients with nonprogressive CLL exhibited significantly higher decorin expression than those with the aggressive type [45]. Similarly, while benign hemangiomas displayed relatively high decorin mRNA levels, the transcription of decorin was completely blocked in malignant vascular sarcomas [46]. Therefore, it seems that malignant behavior and tumor progression may be correlated with the loss of endogenous decorin expression, which may serve as a biomarker for distinguishing between early- and late-stage diseases.

2.2.2 G  enetic Evidence for Decorin as a Tumor Suppressor Further evidence for the antitumor properties of decorin emerged from experimental mouse models where the gene of decorin was unconditionally knocked out [47]. Mice with ablated decorin gene developed spontaneous intestinal tumors when fed with high-fat diet [48]. In this model, loss of decorin resulted in perturbed intestinal maturation including decreased cell differentiation and increased proliferation, which were linked to the downregulation of p21WAF1/CIP1, p27KIP1, intestinal trefoil factor, and E-cadherin, as well as the upregulation of β-catenin signaling [48]. The identification of these signaling molecules paved the way for further research into the antitumor mechanisms exhibited by decorin. Simultaneous genetic ablation of both decorin and p53 led to the formation of aggressive T-cell lymphomas and premature death of these animals [49]. Genetic loss of decorin also resulted in enhanced tumor incidence and tumor count in livers of mice exposed to hepatocarcinogens [36]. Collectively, these studies indicate that the loss of decorin is permissive for tumorigenesis and anticipates its tumor repressive role in cancer.

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2.3

The Structure of Decorin

Structurally, decorin is highly conserved across different species. The mammalian DCN gene is located on chromosome 12q21-q22 and contains eight exons. The synthesis and secretion of decorin chiefly occur in the rough endoplasmic ­reticulum and the Golgi apparatus of fibroblasts, smooth muscle cells, and macrophages [50]. Decorin comprises a 42 kDa protein core with a central domain harboring 12 leucine-rich repeats (LRR) and an N-terminal attachment site for a single glycosaminoglycan (GAG) chain of chondroitin or dermatan sulfate [15, 51] (Fig.  2.1). The 12 LRRs (designated with roman numerals I–XII) form a horseshoe or banana shape [52, 53] with 14 curved β-sheets on the concave surface and α-helices in the convex region [54, 55]. Due to its unique structure, this central domain can interact with a variety of proteins and is responsible for the diverse biological functions of decorin [56]. Although the GAG chain has been shown to be important for some decorin/ligand binding, such as regulating collagen fibrillogenesis [13], most decorin-binding partners interact with decorin at its core protein. Different LRRs possess unique functional properties and contribute to specific bioactivities of decorin. LRRs V–VI constitute the binding site for vascular endothelial

growth factor receptor-2 (VEGFR2) [57] and epithelial growth factor receptor (EGFR) [58]. LRR VII located on the concave surface of the core protein acts as a high-affinity binder of collagen I, the most well-known partner of decorin [59]. LRR XI is known as the “ear” repeat whose truncations or mutations may cause congenital stromal corneal dystrophy [14, 60]. Finally, LRR XII is responsible for the interaction with CCN2/ CTGF [61] (Fig. 2.1a). Albeit decorin forms homodimers in physiological solutions [54] (Fig.  2.1b), monomeric decorin appears to be the active form of the proteoglycan accounting for most of its interactions [62, 63]. Dimerization blocks the central domain, thereby preventing interactions with other substrates. However, recent studies demonstrated that decorin dimerization is reversible and the proteoglycan is able to alternate between the homodimer form and the collagen-binding monomeric form [62]. The same study reports that dimerization is not essential for the stabilization of decorin [62]. By binding to a variety of substrates via its core protein, monomeric decorin can function as a soluble paracrine factor that modulates numerous downstream signaling pathways [64]. Recently, several studies have identified decorin as a substrate of proteases. Matrix metallopro-

Fig. 2.1  The crystal structure of decorin. (a) Cartoon ribbon diagram of monomeric bovine decorin. Β-strands are displayed in yellow, α-helices appear in blue. Leucine-­ rich repeats are designated by roman numerals. LRR V– VI represents the binding site of VEGFR2 and EGFR tyrosine kinases. LRR7 interacts with collagen type I,

while LRR XII is responsible for CCN2/CTGF binding. The ear repeat plays a role in proper folding of the proteoglycan. (b) Dimeric crystal structure of decorin. In physiological solutions decorin exists as a dimer, a form where LRR repeats are hidden, which prevents interactions with most of its substrates. (PDB association number: 1XCD)

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activated protein kinase (MAPK) and NF-κB signaling pathways (Fig.  2.3). These events enhance the secretion of inflammatory factors such as tumor necrosis factor-α (TNFα) and IL-12p70 [71]. Furthermore, interaction with TLR2 and TLR4 can stimulate the production of the proinflammatory programmed cell death protein 4 (PDCD4) by macrophages [68] (Fig. 2.3). Indirectly, by downregulating the bioactivity of transforming growth factor-β1 (TGF-β1), decorin can counteract the transcriptional repression of PDCD4 via inhibition of microRNA-21 (miR-­ 21) [68]. As a consequence, anti-inflammatory mediators such as IL-10 are translationally sup2.4 Roles of Decorin pressed by PDCD4, which creates a proinflammatory tumor microenvironment ([50, 56, 64, 68, in Inflammation 72] (Fig. 2.3). The ability of decorin to regulate and Immunomodulation inflammation is important for understanding its In experiments of tissue stress and injury, it has role in tumor biology, as a proinflammatory become evident that decorin, as well as its rela- tumor microenvironment retards tumor growth tive molecule biglycan, can regulate the innate [7, 50, 64, 73]. In addition, by stimulating CCL2 immune response and inflammatory responses production, decorin recruits mononuclear cells to via TLR2 and TLR4 [56, 67–70] (Figs. 2.2 and the site of injury that results in a sustained inflam2.3). Decorin is able to modulate inflammation matory state [74]. Its interaction with the Class A through a number of mechanisms. It can directly scavenger receptors expressed on the surface of engage TLR2 and TLR4 on the surface of macro- macrophages promotes the adhesion of these phages leading to transient activation of mitogen-­ cells to the matrix [75]. teinase-­ 2 (MMP-2), MMP-3, MMP-7, and membrane type 1-matrix metalloproteinase (MT1-MMP) are all able to cleave decorin, resulting in the inactivation of the molecule [65]. Likewise, BMP-1 peptidase also processes decorin in a similar manner [66]. In addition, decorin can also be inactivated by proteases secreted by inflammatory cells. Decorin as a member of damage associated molecular patterns (DAMPs) may be recognized by pattern recognition receptors such as Toll-like receptor 2 (TLR2) and TLR4 inducing an inflammatory response [67].

Fig. 2.2 Interacting partners of decorin. Growth factors, receptors, and ECM molecules that are regulated by decorin via physical binding with high affinity. Further details are provided in the text

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Fig. 2.3  Immunomodulatory actions of decorin. Decorin engages proangiogenic signaling pathway via activation of TLR2 and TLR4 in concomitant with TGFβ inhibition.

As a result, PDCD4 is transcriptionally activated via inhibition of miR-21 and suppresses the anti-inflammatory protein IL-10

In addition to the effects of decorin on macrophages, decorin can modulate the behavior of leukocytes. In a mouse model of delayed-type hypersensitivity (DTH), Seidler and coworkers reported that decorin mediates DTH responses by

influencing polymorphonuclear leukocyte attachment to the endothelium [76]. This occurs via two mutually nonexclusive mechanisms, a direct anti-adhesive effect on polymorphonuclear leu-

2  Decorin in the Tumor Microenvironment

kocytes and a negative regulation of ICAM-1 and syndecan-1 expression [76]. Despite the ability of decorin to induce proinflammatory responses, decorin core protein was also reported to downregulate chemotactic and inflammatory genes in leukocytes [77]. Thus, it seems that the roles of decorin in regulating inflammation and immune reactions are complex, and further studies are necessary to clarify whether decorin in each specific context initiates pro- or anti-inflammatory responses. Merline and coworkers reported that intact decorin, but not the protein core or GAG chain alone, was able to increase TNFα and IL-12p70 production [68]. Buraschi et al., on the other hand, observed inhibition of anti-inflammatory gene transcription when applying the protein core only [77]. Therefore, it is conceivable that the holomeric molecule and the core protein act oppositely, with the latter competitively binding to TLR receptors and repressing inflammation in the tumor stroma [50]. Decorin knockout animals display proinflammatory phenotype in different in vivo models of fibrotic diseases, while addition of exogenous decorin suppresses inflammation in experimental treatment trials [18, 78–81]. Tumor necrosis factor-α (TNF-α), a major inflammatory cytokine, is a known binding partner of decorin [82]. Upon their interaction, the cytokine is sequestered and prevented from exerting its proinflammatory effects on its receptors.

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explain the mechanism of antitumor action, it has been proposed that collagen-bound decorin sequesters TGF-β and anchors it to the ECM, thereby preventing its interaction with TGF-β receptors on the cell surface [80]. Indeed, decorin exposure inhibits TGF-β signaling in  vitro [64, 85, 86] and indirectly attenuates downstream signaling pathways as reported in a mouse liver fibrosis model [87]. By binding to TGF-β and forming an inactive complex, decorin blocks signaling via Smad2, Smad3, and Erk1/2 and thus curbs fibrogenesis [88]. TGF-β, in turn, inhibits decorin mRNA transcription in fibroblasts [89], which suggests a feedback loop responsible for maintaining the homeostasis of matrix deposition. TGF-β is an important cytokine in the regulation of inflammation, and decorin as a physiological TGF-β-inhibitor limits the duration of TGF-β responses in inflammation and tissue repair [18, 19, 80]. In cancer, the role of TGF-β is contradictory as it can either suppress or promote tumorigenesis, and its mode of action highly depends on the cellular context [90]. Thus, the neutralization of TGF-β by decorin as a potential anticancer strategy needs careful evaluation. Decorin also binds to and inhibits the action of myostatin, another member of the TGF-β superfamily [91, 92] (Fig.  2.2). In this case, decorin sequesters myostatin and attenuates its growth inhibitory effects on myofibers, which results in improved muscle regeneration [91–93]. Myostatin has also been recognized as an important player in the development and maintenance 2.5 Decorin as a Multifaceted of cancer cachexia [94–96]. Myostatin antagonists emerge as promising novel therapeutics Tumor Suppressor: Signaling against cancer cachexia, as they not only prevent in Cancer muscle wasting but may also have a beneficial 2.5.1 Interaction with Growth effect on the overall survival [94–96]. Thus, by Factors quenching the action of myostatin, decorin may be able to antagonize cancer-related wasting. The first growth factor discovered to interact with Via its LRR XII repeat decorin interacts with decorin was transforming growth factor-β (TGF-­ and negatively regulates connective tissue growth β) [83] (Fig. 2.2). Binding of TGF-β by decorin factor (CTGF) [61, 64] (Fig.  2.2). CTGF plays effectively inhibits proliferation of tumor cell important roles in the progression of fibrosis lines dependent on this growth factor and the [97], as well as in the regulation of ECM producspread of cancer [19, 83]. The protein core of tion, chemotaxis, cell proliferation, and differendecorin recognizes and binds to all isoforms of tiation, and also modulates inflammation [98, TGF-β (TGF-β1, TGF-β2, and TGF-β3) [84]. To 99]. CTGF is tightly regulated by TGF-β and

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stimulates decorin expression, suggesting a strict mechanism of autoregulation in this pathway [61]. Wnt-inducible signaling pathway protein-1 (WISP-1, alias CCN4) belongs to the CTGF family and has been identified as an oncogene in a number of cancers, where it enhances cell migration and promotes epithelial-to-mesenchymal transition [100]. Decorin was shown to bind WISP via its dermatan sulfate GAG chain and may thus act as a regulator of Wnt signaling [72, 101] (Fig. 2.2). Decorin is known to downregulate β-catenin [48, 102]; however, a direct connection between decorin binding to WISP-1 and inhibition of β-catenin has not been demonstrated. The oncogenic activin C, another relative of TGF-β has also been recognized as a binding partner of decorin (Fig.  2.2). Their interaction induces caveolin-mediated endocytosis and degradation of the growth factor leading to inhibited proliferation and migration of colorectal cancer cells [103]. Another example of inhibition by direct sequestration is the interaction of decorin with platelet-derived growth factor (PDGF) (Fig. 2.2). In this way, decorin prevents PDGF-dependent phosphorylation of its receptor that results in attenuated signaling in liver cancer [36, 104] and prevents cellular migration in intimal hyperplasia [105]. Decorin plays an important role in the regulation of the insulin signaling pathway. Beside its interactions with IGF-1R and insulin receptors (see later), decorin also binds to IGF-I with low affinity and competes for the growth factor with its endogenous binding partners [64, 106] (Fig. 2.2). In this scenario, the action of decorin is highly concentration dependent, and with increased amounts (e.g., in a therapeutic context), it could effectively attenuate signaling by IGF-I [64]. Although not belonging to the family of growth factors, it is important to note that decorin also binds to a variety of structural components within the ECM, such as different types of collagens, tenascin X [107], and elastin [108] (Fig. 2.2). Importantly, decorin binds to collagen

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VI with high affinity and with matrilin-1 it directly links collagen VI to aggrecan or collagen type II fibrils [109]. These observations highlight the importance of decorin in regulating matrix structure formation and desmoplastic reactions in the tumor stroma [72]. The active contribution of decorin to stromal reaction, a defensive mechanism of the host tissue against cancer, represents another skill in its tumor suppressor repertoire.

2.5.2 D  ecorin Acts as a Pan-RTK Inhibitor The most well-known tumor inhibitory action of decorin is its ability to interact with and directly engage a multitude of receptor tyrosine kinases (RTKs) (Fig.  2.2). This pan-inhibition of RTK pathways earned decorin the reputation of “the guardian from the matrix” [72]. The first RTK discovered to interact with decorin was epidermal growth factor receptor (EGFR), a member of the ErbB receptor family. Detailed analysis in the A431 squamous carcinoma cell line revealed that monomeric decorin binds to a narrow region of the receptor partially overlapping with the binding site of EGF [58, 110–112]. This interaction evokes receptor dimerization and transient autophosphorylation of EGFR leading to caveolin-1-mediated internalization and lysosomal degradation of the receptor [72, 113]. In contrast with phosphorylation induced by the endogenous ligand of EGFR, this interaction elicits cell cycle arrest, apoptosis, angiostasis, and protracted oncogene suppression [9, 64, 72]. The decorin-initiated phosphorylation of EGFR activates the MAPK signaling cascade and subsequent intracellular Ca2+ release [114, 115] and induces the cyclin-dependent kinase inhibitor p21WAF1/CIP1 and cleavage of caspase-­3 [114] (Fig.  2.4). Other studies have also reported inhibition of EGFR and its downstream signaling by decorin. For example, in A549 lung carcinoma cells, overexpression of decorin decreased EGFR activity, which induced G1 phase block and apoptosis of the tumor cells via elevated p53 and p21WAF1/CIP1 expression [116]. Conversely, the lack of decorin resulted in

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Fig. 2.4  Emerging signaling pathways affected by decorin. Antagonism of a multitude of receptors by decorin results in attenuated downstream signaling cascades cul-

minating in antiproliferative and antiangiogenic mechanisms in tumor cells. A detailed description is provided in the text

enhanced EGFR phosphorylation followed by activation of ERK1/2 in experimental models of liver carcinogenesis [36] (Fig. 2.4). Other members of the ErbB family, especially ErbB2 and ErbB4, also interact with decorin [64, 117, 118]. By binding to ErbB4 decorin prevents dimerization of ErbB4 with ErbB2, which arrests growth and prompts differentiation in breast carcinoma cells [117]. Another significant interacting RTK partner of decorin is the Met receptor (hepatocyte growth factor receptor, scatter receptor) [119]. Upon binding to decorin, Met undergoes strong Tyr phosphorylation. Similar to the mechanism seen in the case of EGFR, this provokes the recruitment of c-Cbl and, ultimately, proteasomal degradation of the receptor [9, 119]. Attenuation of Met by decorin suppresses downstream signaling

molecules Myc and β-catenin and thereby inhibits tumor growth [119, 120] (Fig.  2.4). In addition, attenuated activity of Met by decorin induces the antiangiogenic protein TIMP-3 with a concurrent decrease of the proangiogenic proteins HIF-1α and VEGFA [121]. In recent years, many other RTKs were discovered to mediate the bioactivities of decorin. These include IGF-1R, insulin receptor (IR), and their ligands [104, 106, 122, 123]; PDGFRα and its ligand PDGF [36, 104]; VEGFR2 [57, 124]; and MSPR (RON) [36] (Fig. 2.2). As opposed to the general scheme of caveosomal endocytosis and degradation of RTKs upon decorin binding, IGF-1R is not internalized and tagged for destruction. In this case, decorin attenuates signaling of IGF-1R through IRS-1, IGF-1, and Akt/ERK/ p70S6K, resulting in a migratory block. In addi-

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tion, decorin prevents IGF-1 from localizing IGF-1R to caveosomes [28, 72, 123, 125]. It was suggested, however, that decorin inhibits IGF-1R only in cancers but acts as an IGF-1R agonist in normal tissues [56, 106]. A recent study comparing signaling pathways evoked by decorin in four different hepatoma cell lines reported that IGF-1R as well as IR can be either enhanced or inhibited even in cancer cell lines with the same tissue origin [126]. Also, within the same cell line, initial phosphorylation of IGF-1R and IR upon decorin treatment decayed rapidly, and inhibited receptor activity was observed after 2 days. These findings underline that the effect of decorin on IGF-1R regulation is complex and may be affected by unique features (e.g., differentiation state) of the particular tumor [126]. In summary, the interactions between decorin and various RTKs elicit numerous changes in cell signaling pathways with the cumulative effect of attenuated tumorigenesis [64]. Since many solid tumors depend upon RTK signaling, they may be profoundly inhibited by the introduction of decorin [9, 34, 36, 117, 127].

2.5.3 Intracellular Signaling Pathways Activated by Decorin Consequent to the inhibition of receptors, decorin efficiently attenuates downstream signaling pathways involved in tumor cell proliferation, survival, and angiogenesis. Decorin-mediated antagonism of the Met receptor leads to selective degradation of β-catenin and Myc oncoproteins [9, 120] (Fig. 2.4). Binding of hepatocyte growth factor (HGF), the endogenous ligand to Met, initiates a signaling cascade resulting in the stabilization of β-catenin by direct phosphorylation and by simultaneous repression of the function of glycogen synthase kinase-3β (GSK-3β) via phosphorylation [9, 120, 121] (Fig. 2.4). The stabilization of β-catenin seems to occur independently from Wnt signaling and triggers nuclear translocation of β-catenin and transcriptional activation of its target genes such as the oncoprotein Myc driving pro-tumorigenic and pro-survival signals

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[3, 120]. Myc is a transcription factor that coordinates a wide variety of molecules stimulating cell proliferation. One of the targets of Myc is AP4, a transcriptional repressor of the cyclin-dependent kinase inhibitor p21WAF1/CIP1 [128] (Fig. 2.4). Cell cycle suppression through the induction of p21WAF1/CIP1 by decorin was reported in many cancer models and partly accounts for decorin’s ability to attenuate tumorigenesis [72, 129]. As a consequence of Met inhibition by decorin, both β-catenin and Myc are targeted for degradation via the 26S proteasome [56, 120] (Fig. 2.4). As a result, β-catenin fails to translocate to the nucleus and remains in the cell membrane upon decorin exposure [126] (Fig.  2.5). The transcriptional repression and phosphorylation-dependent degradation of Myc (at Thr58 residue) induced by decorin leads to transcriptional activation of CDKN1A locus via loss of AP4 repressor [36, 120] (Fig.  2.4). The increase in phosphorylated Myc and β-catenin may be a result of derepressed GSK-3β downstream of attenuated Met signaling. Although the aforementioned signaling pathway was first described in the case of Met receptor, the same may apply for other RTKs. Downstream of RTKs, the Ras/MEK/ERK, and PI3K/Akt/mTOR the major and best-studied pathways in several types of cancers [130, 131]. The active form of Akt is known to inactivate GSK-3β via phosphorylation [132, 133], which is a key molecule linking several signaling pathways such as those originating from both Wnt and RTKs. Indeed, in an experimental model of hepatocarcinogenesis, ablation of decorin gene resulted in activation of multiple RTKs with enhanced MAPK and Akt pathways, in parallel with decreased degradative phosphorylation of both β-catenin and Myc [36] (Fig. 2.4). Within the nucleus, c-Myc induces AP4 expression that, in turn, represses p21WAF1/CIP1. Reduced p21WAF1/CIP1 levels are insufficient to inactivate CDK4/CyclinD1. In this way, Rb phosphorylation culminates in E2F release, thereby allowing the cell to bypass the restriction point in G1 phase [36] (Fig. 2.4). In addition to p21WAF1/ CIP1 , decorin was shown to upregulate other cyclin-dependent kinase inhibitors such as p27KIP1 [48, 134, 135], p15INK4b [136], and p57KIP2

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Fig. 2.5  Decorin exposure retains β-catenin in the plasma membrane, preventing its signaling within the nucleus. Detection of β-catenin in HepG2 liver cancer cells by immunofluorescence

[137], preventing cell cycle progression from G1 to S phase. Attenuation of β-catenin-driven cyclin D1 expression adds another important component to the cell cycle blockade [72, 120]. While earlier reports all demonstrated that decorin induces cell cycle arrest at the G1/S transition, a recent comparative study revealed that it is also able to induce G2/M arrest in hepatoma cells [126] (Fig. 2.6). The Hep3B hepatoma cell line used in this study harbors deleterious mutations of both p53 and retinoblastoma (Rb) genes [138]; therefore, cell cycle arrest at the G1/S restriction point is compromised, and cells will unconditionally cross the checkpoint. It is a known phenomenon that increased Akt activity may lead to hyper-replication ending up in replication stress that activates the ATR/Chk1/Wee1 system ­stopping the cell cycle at the G2/M transition via phosphorylation of CDK1 [139]. Indeed, in Hep3B cells, the levels of pCDK1 and Wee1 increased in parallel with the high phospho-Akt levels which, in turn, originated from activated IR and IGF-1R; simultaneously, the expression of Cdc25A phosphatase decreased, supporting the mechanism proposed [126]. These experiments demonstrated that decorin not only inhibits the G1/S phase transition but is also capable of blocking cell cycle progression at a later stage at the G2/M checkpoint (Fig. 2.6) [126]. This inhibitory effect of decorin had been unknown prior to this publication, and while more studies are needed to reveal the underlying signaling processes, the newly discovered impact of decorin

on the G2/M checkpoint further substantiates the tumor suppressor ability of decorin. As the p53 and Rb tumor suppressors are among the most frequently mutated genes in cancer, the fact that decorin is able to exert its antiproliferative action even in the absence of functional p53 and Rb adds another argument in favor of its application against cancer. Of note, the very same study proved that decorin evokes completely different cellular responses and signaling pathways in different tumor cell lines, all derived from hepatocellular carcinoma [126]. Therefore, it must be kept in mind that the impact of decorin is highly cell type-specific.

2.5.4 D  ecorin Induces Endothelial Cell Autophagy and Tumor Cell Mitophagy The list of decorin’s antitumor activities was further expanded with the discovery that decorin is able to indirectly induce vascular endothelial cell autophagy resulting in inhibited spread and metastasis of tumor cells (Fig.  2.7). Mechanistically, decorin evokes a prolonged autophagic program via transcriptional induction of Peg3 (paternally expressed 3) tumor suppressor [50, 56, 77]. PEG3, an imprinted gene, is epigenetically silenced via promoter hypermethylation of the active allele in multiple gynecologic and neural tumors [140–143]. Furthermore, Peg3 noncanonically suppresses

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Fig. 2.6  Cell cycle arrest at the G2/M phase provoked by decorin. In the p53 and retinoblastoma double-mutant hepatoma cell line Hep3B, where G1/S arrest is compromised, activation of intracellular signaling pathways (Ras/

the Wnt/β-catenin signaling pathway, partly accounting for the activity of decorin in tumors [9, 144]. In tumors, autophagy acts as an inhibitor of tumor initiation by assisting the clearance of misfolded proteins, reactive oxygen species (ROS), and other factors [50, 145]. After stimulation of autophagy by starvation or mTOR inhibition, decorin binds to VEGFR2 on the cell surface of endothelial cells (Fig. 2.7). Here, in contrast to the pan-inhibition of RTKs in tumor cells, decorin acts as a partial VEGFR2 agonist for initiation of autophagy [9, 146, 147]. The decorin-receptor interaction activates the proautophagic AMPKa/ Vps34 signaling pathway and concurrently inhibits the anti-autophagic PI3K/Akt/mTOR/p70S6K pathway [148, 149]. Importantly, Peg3 is required for decorin-induced transcriptional activation and accumulation of beclin-1 and the microtubule-­

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MAPK and Akt) resulted in enhanced Wee1 expression with concomitant block of Cdc25a. The events ultimately led to inactivating phosphorylation of CDK1 and cell cycle blockade at G2/M

associated protein light chain 3 (LC3) and is responsible for maintaining the basal beclin-1 level in endothelial cells [9, 146]. Collectively, these signaling events culminate in the formation of autophagy precursor complexes including Peg3, beclin-1, and LC3. In parallel, decorin attenuates the formation of the inhibitory Bcl-2/ beclin-1 complex [150] (Fig. 2.7). Of note, EGFR and Akt signaling was reported to inactivate beclin-1 via phosphorylation resulting in autophagy suppression and chemoresistance [151, 152]. Since many RTKs share the same core signaling network, it is plausible that the disengagement of beclin-1 by decorin represents a general process. In addition to initiation of autophagy, decorin also compromises capillary formation [121, 147, 153]. In conclusion, decorin induces autophagy of endothelial cells, reduces the growth of blood

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Fig. 2.7  Schematic illustrations of signaling events in decorin-mediated endothelial cell autophagy (left panel) and tumor cell mitophagy (right panel). For detailed description please refer to the text

vessels in the tumorous stroma, and is able to pre- The interaction stabilized mitostatin mRNA and vent metastasis and spread of tumor cells. led to the accumulation of the protein, and the A novel mechanism of action for decorin in processes were guided by the Met/decorin axis. sustaining angiogenesis and curbing tumorigen- Blockade of mitophagic induction by depletion esis has recently been unfolded, as decorin was of mitostatin compromised decorin’s ability to shown to have a direct impact on catabolic pro- suppress vascular endothelial growth factor A cesses and organelle turnover within the tumor (VEGFA) production (Fig.  2.7). Decorin also proper [56]. Decorin was reported to induce enhances mitochondrial depolarization probably mitochondrial autophagy (mitophagy) via induc- via elevated Ca2+ levels, a consequence of the tion of the Met receptor in a breast carcinoma cell decorin/EGFR interaction [155]. As mitostatin is line [154] (Fig. 2.7). Soluble decorin protein core located at the mitochondrial associated memeffectively inhibited mitochondrial respiratory brane, mitostatin induced by decorin may trigger complexes and mitochondrial DNA (mtDNA) Ca2+ efflux from the endoplasmic reticulum into and initiated a dynamic interplay between per- the mitochondria [56]. oxisome proliferator-activated receptor gamma In conclusion, through modulation of RTK coactivator-1α (PGC-1α) and the decorin-­ signaling decorin stimulates mitophagy, a coninduced tumor suppressor gene, mitostatin [154]. served catabolic process in tumor cells and in

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parallel induces endothelial cell autophagy. These newly elucidated activities of decorin offer additional keys for the control of tumor growth and neovascularization.

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gen IV in the endothelial basement membrane resulting in reduced proliferation of blood vessels [50, 162]. Among the interactions of decorin with RTKs, inhibition of VEGFR2 is the most significant in the attenuation of tumor angiogenesis. Decorin binds to VEGFR2 with high affinity 2.5.5 Further Roles of Decorin at a site partially overlapping with that of in Angiogenesis VEGF-A [19, 57]. Another mechanism whereby decorin exerts its antiangiogenic effect is via Neovascularization is a fundamental step in can- abrogation of the HGF/Met signaling pathway cer progression, whereby the growth of new ves- which culminates in hindered VEGF-mediated sels from preexisting blood vessels supplies the angiogenesis [121, 163]. These events lead to the growing tumor mass. Decorin has been impli- repression of hypoxia-inducible factor-1α cated in the regulation of angiogenesis; its role, (HIF-1α), β-catenin, Myc, and Sp1 (see previous however, remains controversial. The involvement sections). Decorin also stimulates HIF-1α protein of decorin in neovascularization was discovered degradation [121]. Sp1 requires phosphorylation during investigations of corneal development, by the p42/44 mitogen-activated protein kinases where the contribution of decorin to angiogenesis (MAPK, alias ERK1/2) for activation of VEGFA was ambivalent [9, 156, 157]. In a normal, non-­ transcription, which pathway may be suppressed tumorigenic environment, decorin supports by antagonizing RTKs [120, 121]. Within the angiogenesis by promoting integrin-collagen ECM, decorin prevents the release of matrix-­ interaction, thus facilitating endothelial cell bound VEGFA via decreasing the expression and adhesion and migration to type I collagen and activity of MMP-2 and MMP-9 enzymes, which α2β1 integrin [158]. Ablation of decorin gene, in process depends on β-catenin [9, 72, 120, 121]. turn, leads to impaired angiogenesis in the injured In parallel with the inhibition of these proangiocornea [156], and a decorin mimic compound genic factors, decorin induces antiangiogenic promoted proliferation and migration of endothe- molecules such as thrombospondin-1 (TSP-1) lial cells [19, 159]. Furthermore, decorin acts as and tissue inhibitor of metalloproteinase-3 an angiocrine factor (endothelial cell-derived (TIMP3) [121, 164]. Decorin facilitates secretion growth factor for organ-specific tissue regenera- of TSP-1 by interfering with RhoA/ROCK1 sigtion) for liver regeneration after partial hepatec- naling [164], a cascade of many pathways includtomy [160]. Similarly, decorin protected ing those of RTKs. In addition to influencing the endothelia from hyperglycemia and supported balance of anti- and proangiogenic factors, decoangiogenesis via IGF-1R/Akt/AP-1/VEGF sig- rin also utilizes the mechanism of autophagy for naling, which delineates a new therapeutic strat- its antiangiogenic actions (see previous section). egy for patients with diabetic cardiomyopathy Decorin seems to interfere with early events of [161]. Again, the above experiments reporting vascularization by repressing the angioplasticity proangiogenic activity of decorin were all con- of the stroma and by suppressing proangiogenic ducted in normal or non-tumorous models. In factors within the tumor parenchyma [9]. contrast, the greater part of literature on the role of decorin in angiogenesis focuses on tumors and underlines its antiangiogenic role [9, 72]. Decorin 2.6 Therapeutic Approaches was reported to hinder tumor angiogenesis in a variety of tumor cell lines [153], and its expres- In the last decade, a vast number of in vivo studsion is inversely correlated with the extent of ies have shown that decorin administration can tumor vascularization [46]. In addition, decorin inhibit tumor growth and progression. Some earinduces the synthesis of matrix metalloprotein- lier studies were built around the basic idea of ase-­2 (MMP-2), which directly degrades colla- tumor gene therapy, which involves incorpora-

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tion of an anticancer agent into an oncolytic adenovirus [50, 165]. Genetically modified viruses that target and destroy tumor cells without toxicity to normal cells have emerged as a promising strategy [166]. Delivery of decorin via an adenovirus vector into the tumor cells inhibited the growth of lung, colon, and squamous cell carcinomas [167] by attenuating EGFR phosphorylation. Adenovirus-mediated decorin transfer inhibited Met and Wnt/β-catenin signaling pathways and thus prevented the formation of bone metastasis of prostate cancer cells [168]. Forced expression of decorin in osteosarcoma cells resulted in decreased motility and invasion of tumor cells and improved survival of animals [169]. Virus-delivered decorin attenuated breast cancer growth and prevented its metastasis formation in various organs [170–172]. Ma and coworkers found that virus-mediated decorin gene therapy prolonged survival and inhibited tumor growth in an in  vivo glioma model. The rate of inhibition directly correlated with the expression levels of decorin and with the timing of DCN gene transfer [173]. Decorin gene therapy was successfully applied in models of prostate and pancreatic cancers as well [174, 175]. Decorin as a therapeutic gene against cancer may in fact hit multiple birds with one stone. Firstly, the ECM normally impedes the spread of viruses, including therapeutic viral vectors, in the tumor [50, 176]. Secondly, tumors of patients participating in clinical trials are typically characterized by a highly immunosuppressive tumor microenvironment [50, 177]. Decorin as a key organizer of the ECM has the potential to remodel the tumor stroma to enhance viral penetration, and, at the same time, it may also counter intratumoral immunosuppression by antagonizing TGF-­ β. Indeed, intratumoral injection of Ad-DCN, where a mutant decorin gene with increased binding affinity to collagen was applied showed greatly enhanced tumor penetration and led to improved tumor reduction and survival benefit [178]. In these experiments, increased cancer cell cytotoxicity was achieved through the action of decorin on the ECM.  The same vector injected into the primary tumor greatly reduced the lung metastases formation of melanoma [178]. In a

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recent study, decorin was applied as an adjuvant to conquer TGF-β-mediated immunosuppression of tumors. To this end, a novel oncolytic adenovirus coding for IL-12 (a potent antitumor cytokine) and decorin was created [179]. Treated tumors showed significantly higher levels of interferon (IFN)-gamma, tumor necrosis factor-­ alpha, monocyte chemoattractant protein-1, and IFN-γ-secreting immune cells. Also, the vector attenuated intratumoral TGF-β expression, promoted infiltration of CD8+ T cells, and enhanced viral spread within the tumor [179]. The combined utilization of decorin with a cytokine to overcome tumor-induced immunosuppression may be a promising future avenue of cancer immunotherapy. In addition to in  vivo gene therapy studies, several successful in  vitro and in  vivo experiments utilizing recombinant decorin have been reported. Administration of decorin core protein to A431 squamous carcinoma cells and transfection of DCN cDNA into breast carcinoma cells inhibited EGFR activity, induced apoptosis, and hindered tumor growth [112, 115, 118]. Systemic administration of decorin inhibited the growth and metabolism of breast cancer cells and interfered with their metastatic spread to the lungs [114, 117]. Decorin was shown to synergize with carboplatin in inhibiting the proliferation of ovarian tumor cells [180] but attenuated the cytostatic effect of carboplatin and gemcitabine on pancreatic cancer cells [44]. In both models, as expected, decorin exerted an antiproliferative effect on tumor cells. These studies highlight the tissue specificity of decorin’s action and remind that in clinical settings, the ability of decorin to modulate the efficiency of chemotherapeutics must be taken into account. Despite its lack of toxicity and the treatment success seen in a large number of preclinical studies as well as in in vivo anticancer and antifibrotic experiments, decorin has not yet been developed into a clinical drug. A major reason for that is that decorin as a proteoglycan is difficult to mass produce, since due the heterogeneity of its GAG chain, recombinant decorin is inhomogeneous in size and hence does not meet the criteria for human drugs [19]. The GAG chain,

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however, is dispensable for most of its tumor suppressor effects, and the majority of its interactions with growth factors or RTKs occurs via binding to the protein core. Thus, synthesizing decorin without its GAG chain may be a viable option [19], and these manufacturing issues can be simply solved by site-directed mutagenesis [19, 51]. In preparation for a future clinical development, the recent years have seen multiple efforts to improve the biological activity of decorin and thus make it more attractive as a therapeutic drug. A systemically administered, targeted version of decorin core protein was developed [19, 80, 81, 181], where the enhanced core protein is obtained by fusion to a small peptide acting as an address tag that delivers decorin to inflammatory and angiogenic vasculature [80, 81]. The small peptide tag named “CAR” (after its sequence CARSKNKDC) selectively homes to neoangiogenic blood vessels of tumors and regenerating tissues [19, 81, 182] and can efficiently deliver therapeutic amounts of decorin. Beside its tissue specificity, the CAR-DCN fusion protein displays enhanced inhibition of TGF-β-stimulated tumor cell proliferation and spreading [81, 183]. The CAR peptide targets only the inflammatory but not the normal vasculature in lung diseases and is suitable for delivery of pharmaceutical agents in an organ-specific fashion [182, 184– 188]. These improvements in delivery and activity strengthen the rationale for applying decorin in the treatment of cancer and other pathological conditions related to angiogenesis or inflammation [19].

2.7

Conclusions and Future Perspectives

The tumor microenvironment plays a determining role in cancer development by regulating multiple processes between the extracellular matrix and tumor cells. Decorin, a prototype member of the SLRP family found in a variety of tissues, has gained recognition for its essential roles in inflammation, fibrotic disorders, and cancer. Originally discovered as a collagen-bound

molecule regulating fibrillogenesis, decorin has emerged as a multifunctional and multifaceted signaling molecule whose activity repertoire is far beyond being a mere structural component of the stroma. Studies on mice with ablated decorin gene revealed that the lack of decorin is permissive for tumor development. On the same note, reduced expression or abrogation of decorin was observed in several types of cancer, suggesting that decorin tends to act as a tumor suppressor in these contexts. Moreover, when applied as a therapeutic agent, decorin effectively inhibited tumor formation, progression, angiogenesis, and metastasis in a multitude of experimental models, which raised substantial interest in decorin for clinical medicine. The antitumor activity of decorin relies on targeting a wide selection of binding partners such as growth factors, cell surface receptors, and extracellular matrix components, and its combined effects amount to potent inhibition of downstream signaling pathways of cell proliferation, migration, and angiogenesis. Through a novel regulatory mechanism, decorin was also shown to induce conserved catabolic processes such as endothelial cell autophagy and tumor cell mitophagy. In summary, while the biological action of decorin is irrefutably complex, all its interactions lean toward curbing tumor progression. The antitumor effect is achieved partly by directly modulating tumor cells and partly by influencing the surrounding tumor microenvironment that would otherwise promote malignant transformation and tumor progression. Therefore, decorin as “a guardian from the matrix” may be a valuable tool in combatting cancer, especially those cancer types that heavily depend on RTK signaling. The future of utilizing decorin in medicine is laden with challenges. As decorin is recognized and cleaved by multiple proteases, decorin can easily be inactivated in pathological processes. Thus, strategies of delivering intact and functional decorin protein core in therapeutically relevant quantities are on quest, and enhancements such as tissue targeting or improved efficacy will be welcome. Identification and isolation of leucine-­rich repeats of decorin exhibiting distinct bioactivities could be of great interest, as engi-

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neering these fragments could allow specific targeting of receptors and could be developed into adjuvant peptide therapy. Further studies should also elucidate the functional interaction between decorin and existing anticancer chemotherapeutics to reveal possible limitations of its use. For basic research, the interactome of decorin suggests many unexplored abilities and possibilities. Future investigations may identify novel binding partners and signaling pathways or reveal previously unknown connections between cellular processes. Furthermore, as decorin is a member of the SLRP family, it is conceivable that other SLRPs share interacting partners with decorin and thus create a complex dynamic signaling network. Our increasing understanding of the interactome of decorin is only one of the recently revealed mysteries of the extracellular matrix that assists in expanding our comprehension of molecular and cellular oncology.

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3

Syndecan-1 in the Tumor Microenvironment Adriana Handra-Luca

Abstract

3.1

Syndecan-1 along with the other three syndecan proteins is present in the varied components of the tumor microenvironment: fibroblasts, inflammatory tumor immunity-­ associated cells, vessels, and extracellular matrix. Epithelial and non-epithelial tumors may show stromal syndecans. The main relevance of stromal syndecans as tumor biomarker resides in the relationships to tumor features such as type and differentiation as well as to prognosis.

This chapter focuses on syndecan-1  in tumor microenvironment components as detected by immunohistochemistry. Data on the other three syndecans are also presented. The relevance for tumor biology and prognosis is discussed.

Keywords

Syndecan-1 · Tumor · Stroma · Microenvironment · Pathology · Microscopy · Immunohistochemistry · Fibroblast · Vessels · Inflammatory cells · Extracellular matrix · Adenocarcinoma · Squamous cell carcinoma · Morphotype · Prognosis

A. Handra-Luca (*) Service d’Anatomie pathologique; APHP GHU Avicenne, University Sorbonne Paris Nord, Bobigny, France e-mail: [email protected]

3.2

Introductory Section

Tumor Microenvironment

The tumor microenvironment is mainly represented by the tumor stroma. The components of tumor stroma are the fibroblasts and myofibroblasts, the inflammatory cells (frequently lymphocytes and plasmocytes), the blood vessels, and the extracellular matrix (tumor-associated interstitial tissue or connective tissue). The tumor microenvironment is easily detected in epithelial tumors when abundant. In mesenchymal and hematopoietic tumors, the tumor microenvironment is scant, represented mainly by reactive inflammatory cells and vessels. The majority of studies with regard to syndecans and tumor microenvironment focus on expression of these molecules in stromal fibroblasts and extracellular matrix. A pleiotropic role for syndecan-1 is suggested as determined by the cellular origin, location, and type: for example, tumor promoter when stromal and tumor suppressor when epithelial [8].

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 A. Birbrair (ed.), Tumor Microenvironment, Advances in Experimental Medicine and Biology 1272, https://doi.org/10.1007/978-3-030-48457-6_3

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Fig. 3.1  In colon adenocarcinoma, syndecan 1 (as detected by the anti-CD138 antibody, clone MI15 Leica Biosystems) was expressed heterogeneously in the epithelial tumor component and in the stroma (asterisk white and black, respectively, gray asterisk for extracellular, stromal mucus). Immunohistochemistry: Leica system microscopy photos, original magnification ×5 (a, b)

Varied tumor types have been studied for syndecans in the tumor microenvironment, with epithelial tumors being the most frequently reported. Representative examples for syndecan-1 expression in the tumor stroma are shown in Figs. 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, and 3.9. Among epithelial tumors, glandular-type carcinomas of the gynecological and gastrointestinal systems are the most studied. Breast carcinomas have been extensively studied for stromal syndecans [4, 7, 9, 11, 12, 14, 15, 19, 20, 28]. Other studied

gynecological system carcinomas are those of the endometrium [10] and of the ovary [8]. There are also reports of syndecans in gastric [32], colorectal [20], and hepatic tumor tissues [26]. Among the squamous cell carcinoma-­ type epithelial tumors, most reports are with regard to the tumors located in the head and neck [22] and oral regions [1] and of the skin. Lung non-small cell carcinomas [3, 20] have also been studied for syndecans as well as transitional-type epithelial tumors such as urinary bladder carcinomas [20, 31]. Less fre-

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Fig. 3.2  In colon adenocarcinoma, syndecan-1 (as detected by the anti-CD13s antibody, clone MI15 Leica Biosystems) in lympho-plasmocytes at proximity to tumor glands (black arrows) as well as in stromal fibroblasts. Endothelial cells did not express CD138 (white arrow). Immunohistochemistry: Leica system microscopy photos, original magnification ×5 (a) and ×40 (b)

quent tumors such as salivary gland tumors may also express stromal syndecan-1 [2]. Syndecans are also expressed in the stroma of multiple myeloma, a non-epithelial tumor [5]. The fibroblast is considered the most important cell type in the tumor stroma. The fibroblast synthetizes, organizes, and maintains the 3D network of glycoproteins and proteoglycans known as the extracellular matrix [33]. Carcinoma-­ associated stromal fibroblasts are supposed to have an activated phenotype, pointed out by

expression of smooth muscle markers (suggestive for contractile proteins), by an enhanced proliferative and migratory potential, by an altered gene profile, and by a contribution to an altered extracellular matrix architecture [33]. Syndecan expression in stromal components is considered the result of both tumor induction and tumor shedding/shift [19, 20, 30]. Shed syndecans is considered to retain biological activity [27]. The use of varied syndecan-1 clones for immunohistochemistry, that is, a

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Fig. 3.3  In gallbladder adenocarcinoma, syndecan-1 (as detected the anti-CD138 antibody, clone MI15 Leica Biosystems) was expressed heterogeneously by stromal lympho-­ plasmocytes; black arrows). Endothelial cells also expressed CD138 (white arrow). Immunohistochemistry: Leica system microscopy photos, original magnification ×10 (a) and ×20 (b)

clone against the cytoplasmic domain (2E9 clone) and a clone against the ectodomain (IC7 clone), suggests a possible stromal synthesis of syndecan-1 as reported for human ovarian tumors [8]. Further, syndecan-1 induction in fibroblasts is required for their mitogenic effect on breast carcinoma cells [30]. Elevated syndecan-1  in mesenchyme-­ derived stromal/ tumor-associated fibroblasts and decreased level in the epithelial cells in infiltrating carcinoma, as observed on tissue sections, resemble

to what observed during embryonal morphogenesis for the condensed mesenchyme. These expression patterns in mammary carcinomas may be considered as an oncofetal reactivation of an epithelial-mesenchymal pathway in tumors [19]. Further, syndecan-1-positive fibroblasts induce carcinoma cells to form branching irregular clusters resembling infiltrating carcinoma. For the epithelial-stromal signaling, direct carcinoma cell-fibroblast contact is required. The diffusion range of released syn-

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Fig. 3.4  In gallbladder adenocarcinoma, syndecan-1 (as detected by the anti-CDI38 antibody, clone MI15 Leica Biosystems) was expressed by endothelial cells. Stromal fibroblasts also expressed (heterogeneously) CD138. Tumor cells expressed strongly and diffusely CD138 (white asterisk). immunohistochemistry: Leica system microscopy photos, original magnification ×10 (a) and ×20 (b)

decan-1 ectodomain may be limited by binding to pericellular matrix components. The disruption of syndecan-1 shedding could be an opportunity for therapeutic intervention. Of interest would be that syndecan-­1  in breast carcinoma stromal fibroblasts promotes the assembly of an architecturally abnormal extracellular matrix permissive for breast carcinoma directional migration and invasion [33]. As suggested by coculture experimental models, syndecan-1  in

stromal fibroblasts stimulates breast carcinoma growth and ­angiogenesis and is considered to alter extracellular matrix composition and architecture. Altered extracellular matrix fiber architecture promotes directional migration of breast carcinoma cells [33]. Of note would be that genotoxic radiation can provoke premature senescence in breast stromal fibroblasts. An autocrine TGF-beta loop is formed leading to syndecan-1 overexpression [13].

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44 Fig. 3.5  In lung squamous cell carcinoma (non-small cell carcinoma). syndecan-1 (as detected by the anti-CDI38 antibody, clone MI15 Leica Biosystems) was heterogeneously expressed in the tumor and stromal components (white and black asterisks). CD138 was expressed strongly in lympho-plasmocytes (black arrows) while moderately in stromal fibroblasts (gray arrow). Immunohistochemistry: Leica system microscopy photos, original magnification ×5 (a) and ×40 (b)

3.3

Syndecan-1 and Tumor Microenvironment: Detection Methods

The main and most important detection method for syndecans in tumors is immunohistochemistry. This technique, performed on tissue sections, allows the detection of syndecan protein expression not only in tumor cells but also in the tumor microenvironment components: fibroblasts, endothelial cells, inflammatory cells, and vessels.

Cellular syndecan protein expression location may be evaluated whether nuclear, cytoplasmic, or membrane. The type of intracellular expression can also be assessed: cytoplasmic dot-like or diffuse, membrane, continuous or discontinuous. Nucleolar staining can also be detected [26]. Most reports in the medical literature focus on tissue expression of syndecan-1. Antibody clones used for the detection of syndecan-1 are B-B4 clone (detecting the syndecan-1 ectodomain) [1, 4, 6, 8, 9, 12, 14, 20, 22, 28, 32], MII5 [17, 23, 31],

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Fig. 3.6  In lung squamous cell carcinoma (non-small cell carcinoma), syndecan-1 (as detected by the anti-CD138 antibody, clone MI15 Leica Biosystems) was heterogeneously expressed in endothelial cells (white arrow). Immunohistochemistry: Leica system microscopy photos, original magnification ×5 (a) and ×40 (b)

B-A38 [3], 2E9 (detecting the epitope of the cytoplasmic domain of syndecan-­1 and syndecan-3) [26], C-20 [18], 281–2 [19], M7228 [16], JASY1 [11], DL101 [21], and monoclonal IgG1 Santa Cruz [15]. The other syndecan proteins can also be detected on tumor tissue sections by immunohistochemistry. The clones used for the detection of syndecan-2 are 10H4 [26] and ZMD308 [1]. The clone used for the detection of syndecan-3 is IC7 (detecting the ectodomain), while the clones used for d­ etection of syndecan-4 are 8G3 [8, 26]

and 5G9 [12]. The fibroblast morphotype is confirmed by the presence of other proteins such as vimentin and alpha-smooth muscle actin and by the lack of cytokeratin [19, 20]. Other methods of tissue in situ detection of syndecan molecules in human tumors are immunofluorescence staining [12, 16, 18, 29], in situ hybridization [20], and immunoelectron microscopy [26]. Flow cytometry can also be used [5]; however, this method does not allow an intratissue localization of syndecan molecules.

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46 Fig. 3.7  In pancreatic solid and pseudo papillary neoplasia, syndecan-1 (as detected by the anti-CD138 antibody, clone MI15 Leica Biosystems) was expressed heterogeneously in the extracellular matrix (black asterisk). The tumor cells expressed strongly syndecan-1 (white asterisk) as well as stromal fibroblasts (gray arrow). Immunohistochemistry: Leica system microscopy photos, original magnification ×2.5 (a) and ×20 (b)

3.4

Syndecan-1 and Tumor Microenvironment: Fibroblasts

Syndecan-1 is expressed by stromal fibroblasts in 9.3–83% of human breast carcinomas [4, 7, 9, 11, 12, 14, 15, 19, 20, 28]. Tumoral stromal syndecan-1 is significantly increased as compared to normal breast tissue [15]. Benign tumors such as fibroadenomas and cystadenoma phyllodes do not show stromal fibroblast syndecan-1 [28]. In

the series reported by Barbareschi et al. [4], stromal expression of syndecan-1 (fibroblast and stroma syndecan-1) was heterogeneous. In 129 of the studied tumors, more than 10% of the stroma was stained. A complete lack of syndecan-1 expression was detected in 37% of the breast carcinomas, while a strong intensity staining was detected in 32%. The most intense staining was in those tumors with dense desmoplastic stroma. In the series reported by [20], stromal syndecan-1 was of increased intensity as compared to

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Fig. 3.8  In multiple myeloma, syndecan-1 (as detected by the anti-CD138 antibody, clone MI15 Leica Biosystems) was not expressed in the stromal vessels (white arrows). One CD138-positive plasmocyte was observedd in a vessel lumina. Tumor cells expressed diffusely and strongly CD138. Immunohistochemistry: Leica system microscopy photos, original magnification ×20 (a, b)

tumoral, epithelial syndecan-1. More recently, Kind et al. [11] detected, in the 58.1% of tumors positive for stromal syndecan-1, two “stromal syndecan-1” patterns: peritumoral and diffuse. In the series reported by Lendorf et  al. [12], the highest stromal cell syndecan-1 expression was in invasive carcinomas, ductal and lobular. In the latter series, stromal syndecan-1 was confined to fibroblasts adjacent to tumor cells [12]. Of note would be that in breast carcinomas, increased stromal syndecan-1 is also detected after ionizing

radiation. Increased syndecan-1 appears also at periphery of senescent Sudan Black-positive (senescent) cells [13]. In human endometrial cancer [10], another type of glandular-type carcinoma, stromal syndecan-1 expression is of heterogeneous intensity. More than half of endometrial cancers show strong or moderate expression. In human ovarian tumors [8], stromal syndecan-1 is detected not only in benign tumors but also in borderline and malignant tumors. However, the highest stromal

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Fig. 3.9  In thyroid and parathroid (A and B, respectively) adenomas, syndecan-1 (as detected by the anti-CD138 antibody, clone MI15 Leica Biosystems) was expressed heterogeneously by stromal lympho-­ plasmocyte (black arrows). Endothelial cells and fibroblasts did not express syndecan-1 (gray and white arrows, respectively). Immunohistochemistry: Leica system microscopy photos, original magnification ×20 (a) and ×10 (b)

syndecan-­1 expression intensity is seen in invasive adenocarcinomas, confined to cells adjacent to invasive carcinoma. In human gastric cancer [32], stromal syndecan-­1 is relatively rare (9%). Stromal expression of syndecan-1 correlates with decreased epithelial syndecan-1. A more recent report by Charchanti et al. [6] indicates that stromal moderate and high syndecan-1 is detected in most gastric tumors, 99 of the 104 tested tumors. The different results between the two studies are pos-

sibly related to different evaluation methods. In human colorectal carcinomas [20], another example of digestive system tumors, diffuse stromal syndecan-1 expression can be observed. However, in the series reported by Mitselou et al., stromal syndecan-1 was detected in 56% of the tumors [21]. Syndecan-1 is not detected in stromal fibroblasts of pancreas carcinomas [20]. In human lung non-small cell carcinomas [3], comprising a mixture of histological types including adenocarcinomas and squamous cell

3  Syndecan-1 in the Tumor Microenvironment

carcinomas, CD138-positive cells are more abundant in the stroma than in the tumoral epithelium. A cutoff of >25% positivity in the stroma is considered as significant. To mention would be that stromal CD138 positivity correlates to intraepithelial CD138 positivity. In human oral carcinomas [18], stromal syndecan-­1 is observed in approximately one-third of the tumors. Syndecan-1 may be detected in various stromal areas, some adjacent to the tumor cells, while others at distance. In the series reported by Mukunyadzi et al. [22], more than two-thirds (74%) of the tumors show stromal fibroblast expression. The intensity of stromal fibroblast syndecan-1 is increased for tumors with a “less cohesive” invasion pattern type (invasion as single or small groups of cells) as compared to those with a “broad cohesive” invasion pattern type. Syndecan-1 is also expressed in the stroma of rare head and neck tumors, in both benign and malignant salivary gland tumors [2]. Thirty-three percent of pleomorphic adenomas and 26% of Warthin tumors (both studied cases of basal cell adenoma and myoepithelioma) as well as 54% of adenoid cystic carcinomas and all cases of acinic cell carcinomas expressed stromal syndecan-1. The expression intensity was low in pleomorphic adenomas, basal cell adenomas, and myoepitheliomas. In human urinary bladder carcinomas [31], stromal syndecan-1 may be detected in less than half of the tumors. Mennerich et al. [20] reported stromal syndecan-1 expression decreasing in intensity with the increase of the distance from the tumor cells. Prostate carcinoma may also show syndecan1-positive cells in the stroma, in the PCSP (prostate cancer syndecan-1-positive) cells (29). In myeloma, a non-epithelial tumor type, syndecan-­1 is not related to fibroblasts [5]. The second type of syndecan molecules, syndecan-­2, is confined to stromal cells in human benign, borderline, and malignant tumors [8]. Senescent fibroblasts in human breast carcinomas after ionizing radiation show unaltered syn-

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decan-­2 [13]. In human liver carcinomas, syndecan-2 is expressed in mesenchymal stromal cells [26]. In human oral and cutaneous squamous cell carcinomas [1], syndecan-2 is expressed in the cytoplasm of stromal cells/fibroblasts in approximately half of both tumor types (50% and 56% of oral and cutaneous squamous cell carcinomas, respectively). With regard to syndecan-3, there are few data reported for human tumors. In liver carcinomas [26], syndecan-3 is expressed in tumor mesenchymal stromal cells of hepatocellular carcinoma and cholangiocarcinomas. Syndecan-4 is not expressed in breast carcinoma stromal fibroblasts [19] nor in ovarian epithelial tumors [8]. However, senescent fibroblasts after ionizing radiation in human breast carcinoma show syndecan-4 overexpression [13]. Moreover, 26% of the breast carcinomas showed stromal syndecan-4 [12]. The differences in the number of studied tumor may be one explanation for the different results.

3.5

Syndecan-1 and Tumor Microenvironment: Inflammatory Cells

Syndecan-1 is expressed by tumor immunity-­ associated lymphoplasmocytes in human lung non-small cell carcinomas [16] as well as in human oral and skin squamous cell carcinomas. In the latter two tumor types, syndecan-1 is expressed in the stromal inflammatory cells in approximately half of both tumor types (52% oral and 48% cutaneous squamous cell carcinomas) [1]. The number of syndecan-1-positive inflammatory cells is higher in oral squamous cell carcinomas than in cutaneous squamous cell carcinomas [1]. Syndecan-2 is expressed in inflammatory cells in less than 25% of both oral and cutaneous squamous cell carcinomas (14% and 22%, respectively) [1]. Syndecan-4 is not expressed in tumor stromal immune cells in the tested ovarian tumors [8].

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3.6

Syndecan-1 and Tumor Microenvironment: Vessels

Syndecan-1 is reported in small blood vessels of breast tumors [12]. Syndecan-1 is also expressed in the endothelial lining of stromal vessels of gallbladder adenocarcinoma and lung squamous cell carcinoma (Figs. 3.3, 3.4, 3.6, and 3.7). With regard to the other syndecan proteins, syndecan-2 is expressed weakly in vessels of hepatocellular carcinomas and cholangiocarcinomas. The precise location, whether endothelial or pericyte, is difficult to determine even at high-­ magnification optical microscopy [26]. In ovarian tissues, syndecan-3 expression in tumor stromal vessels is stronger than in normal ovarian vessels. Syndecan-3 can be detected in the vessel walls of the stroma of benign and malignant ovarian tumor [8] and in breast tumor microvessels [19].

3.7

Syndecan-1 and Tumor Microenvironment: Extracellular Matrix

To mention would be that, in several studies, fibroblast and extracellular matrix syndecan-1 are considered together as “stromal syndecan-1.” However, only extracellular matrix expression of syndecan-1 is reported in human breast carcinomas [28]. Benign tumors do not show this expression pattern. The stromal collagen in the majority of human oral squamous cell carcinomas (91%) and in approximately half of cutaneous squamous cell carcinomas (48%) shows syndecan-1 expression [1]. The number of syndecan-1-positive collagen fibers is higher in oral than in cutaneous squamous cell carcinomas [1]. In multiple myeloma [5], a non-epithelial tumor type, the fibrotic stroma unrelated to fibroblasts, stains intensely for syndecan-1. The aspects of the syndecan-1 extracellular matrix accumulation are likely derived from shedding of the protein. In the extracellular matrix, syndecan­1 may form a reservoir of growth factors that drive reemergence of tumor after treatment [5].

3.8

Syndecan-1 and Tumor Microenvironment: Clinical Relevance

3.8.1 Tumor Features and Syndecan-1 in the Tumor Microenvironment The relationships between stromal syndecan-1 and tumor features and outcome favor the hypothesis of a pro-oncogenic role of stromal syndecan-1. With regard to breast cancer, stromal peritumoral and diffuse stromal CD138 relate significantly to histological tumor type, to TNM stage components, to histological grade, to estrogen and progesterone positivity, and to triple negativity [11]. In the series reported by Loussouam et al. [17], stromal syndecan-1 relates to a high Elston-Ellis grade, while in the series reported by Lendorf et al. [12], stromal syndecan-1 correlates to tumor grade and type. In human endometrial cancer, stromal syndecan-1 is associated to a high FIGO grade [10]. Stromal expression of syndecan-1  in digestive, gastric carcinomas correlates with decreased epithelial expression as well as with an epithelial intestinal tumor histotype and with Borrmann type 1 [32]. In the series of gastric tumors reported by Charchanti et  al. [6], a low stromal syndecan-1 (50 cell types, [57, 58] (Fig. 5.2). OPCs are highly proliferative, including chondroblasts, osteoblasts, keratino- migratory bipolar cells that are distinct from neucytes, smooth muscle (SM) cells and microglia/ rons, mature oligodendrocytes and astrocytes, or macrophages [10, 52, 53]. Up- and down-­ microglia. They are precursors to oligodendromodulation allows it to be expressed by imma- cytes but may differentiate into protoplasmic ture progenitor cells in developmental lineages of astrocytes in the grey matter [59]. OPCs express epithelial and mesenchymal origin [10, 17, 38]. It platelet-derived growth factor receptor alpha is not expressed by multipotent stem cells, but it (PDGFRα) [60], A2B5 [61] and 2′,3′-cyclic is upregulated in stem cells during the initial nucleotide phosphodiesterase (CNPase) [62]. phases of commitment to a particular cell linCSPG4/NG2 promotes OPC proliferation and eage. Partially committed progenitors that are migration by acting as a co-receptor for the core still proliferative and motile and have retained a protein-binding growth factors PDGF-AA and degree of developmental plasticity express high FGF-2 [25, 45, 63] and contributes to their polarlevels of CSPG4/NG2 until their terminal differ- ity [64]. In particular, it regulates PDGF signalentiation, when CSPG4/NG2 is turned off. The ling by interacting with PDGFRα, which expression of CSPG4/NG2 on immature progeni- mediates OPC proliferation in response to its tor cells indicates its contribution to processes ligand PDGF [60] and EGFR signalling [65]. Fig.  5.1 (continued) transmembrane domain (amino acids 2222-2246) and a short C-terminal cytoplasmic domain (amino acids 2247-2322). The extracellular ectodomain contains three subdomains: domain 1 (D1), domain 2 (D2) and domain 3 (D3). D1 is a N-terminal globular domain (amino acids 1-640) stabilised by intramolecular disulphide bonds that contains two laminin G-type motifs (L1 and L2) and abundant disulphide bonds critical to maintain the tertiary structure. D2 is a central domain (amino acids 641-1590), containing 15 “CSPG repeat” motifs that are the attachment sites for CS chains, collagens II, V and VI.  D2 interacts with integrins and ECM proteins and binds and presents growth factors to

receptor tyrosine kinases. D3 is a juxtamembrane globular domain (amino acids 1591-2221) containing binding sites for galectin-3 and β1 integrins and proteolytic sites for CSPG4/NG2 cleavage. The cytoplasmic tail interacts with different proteins and functions as a phosphoacceptor site for the extracellular signal-regulated kinase 1/2 (ERK1/2). The PDZ domain is involved in protein scaffolding functions. CSPG4/NG2 activates two major cellular signalling cascades: the mitogen-activated protein kinase pathway, through the receptor tyrosine kinase-ERK1/2 axis, and the integrin/focal adhesion kinase (FAK) pathway. Through these pathways, CSPG4/NG2 ultimately promotes tumour progression across a variety of cellular functions [127]

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Fig. 5.2  Summary scheme of CSPG4/NG2 expression during neurogenesis and gliomagenesis and its functions in normal and pathogenetic mechanisms of the central nervous system (CNS). CSPG4/NG2 expression is detectable in subsets of normal glial cells in developing and adult CNS. It is not expressed by multipotent stem cells but is upregulated in the NG2-glia and in the partially committed oligodendrocyte precursor cells (OPCs) that

are still proliferative and motile. Upon terminal differentiation of these progenitors into mature oligodendrocytes, CSPG4/NG2 is downregulated. It is once again upregulated in pathological conditions including malignant tumours. CSPG4/NG2 aberrant expression has been associated with gliomas where it affects tumour cell adhesion, migration, proliferation, chemoresistance and neo-­ angiogenesis [127]

CSPG4/NG2 is not expressed by multipotent neural stem cells in the primary and secondary germinal zones of the CNS, but it is upregulated in progenitors that originate in these zones and are committed to the oligodendroglial lineage [7] (Fig. 5.2). OPCs originate in the neuroepithelium of the spine and spread to populate the brain and spinal cord as a result of three waves of production and migration and may myelinate the entire CNS [66, 67]. They are uniformly distributed in the grey and white matter of the mature CNS:

white matter OPCs proliferate and contribute to adult oligodendrogenesis, whereas grey matter OPCs are slowly proliferative or quiescent cells that mainly remain in a state of immaturity [68]. OPCs differentiate into less mobile, pro-­ oligodendrocytes that further differentiate into oligodendrocytes. The process of maturation until terminal differentiation is characterised by morphological changes and the expression of cell surface markers that are specific to the stage of differentiation. The expression of CSPG4/NG2

5  Chondroitin Sulphate Proteoglycans in the Tumour Microenvironment

and PDGFRα is lost in favour of myelin basic protein (MBP), proteolipid protein (PLP) or myelin-associated glycoprotein (MAG) [69]. Adult polydendrocytes have intimate spatial relationships with synaptic structures and nodes of Ranvier, receive functional synaptic input from glutamatergic neurons [70] and act as a source of new oligodendrocytes for the remyelination of demyelinated axons [71]. Accordingly, their number increases at sites affected by injury, inflammation, demyelination or remyelination [72, 73]. CSPG4/NG2 may be a reliable marker of adult oligodendrocyte progenitors under normal, reactive or inflammatory conditions [72–74].

5.4

 SPG4/NG2 in Human Brain C Tumours

5.4.1 T  he Role of CSPG4/NG2 in the Origin of Gliomas The findings of basic and clinical experimental studies suggest that CSPG4/NG2-expressing OPCs may contribute to the origin and progression of human gliomas [8, 75–79]. Gliomas are the most frequent primary tumours of the CNS and are classified by the 2016 revised World Health Organisation (WHO) classification of CNS tumours into histologically and genetically identified entities and variants [80]. GB is the most frequent tumour in adults and is characterised by phenotypic and genotypic heterogeneity, resistance to therapy and recurrence. Despite major therapeutic advances, its prognosis remains poor because of the lack of local control and infiltrative growth pattern. GB is characterised by aberrant signalling pathway activation [81–84], including abnormal RTK signalling through the MAPK cascade and abnormal integrin signalling following FAK activation. As CSPG4/NG2 participates in the extracellular availability of oncogenic factors and receptors, it may be responsible for aberrant RTK activity driven by alterations in receptor expression or altered ligand availability.

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CSPG4/NG2-expressing OPCs participate in the development of adult gliomas [64, 85] (Fig. 5.2), and may give rise oligodendroglial or astrocytic tumours upon Ras activation and p53 depletion [86]. CSPG4/NG2, PDGFRα and Olig2 (all common markers of OPCs) are overexpressed in pilocytic astrocytomas and diffuse gliomas (oligodendrogliomas or astrocytomas) and heterogeneously expressed in GB [85, 87–89]. Aberrant activation of the PDGFRα signalling pathway has been described in malignant astrocytomas [82, 90]. PDGFRα/PDGF-AA overexpression, EGFR gene amplification and isocitrate dehydrogenase 1 (IDH1) mutations are the main genetic alterations found in low-grade gliomas and secondary (IDH-mutant) GBs, with high PDGFR-AA expression levels and gene mutations being frequent in the Proneural subtype of GBs [84]. Moreover, CSPG4/NG2 is involved in the EGFR-PI3K-AKT signalling pathway, where it enhances the activation of the EGFR TK domain and, therefore, the proliferative capability of GB cells [91]. Despite its variable expression in human gliomas (Fig. 5.3), CSPG4/NG2 correlates with the malignancy grade [7, 16, 92, 93]. Between 50% and 67% of GBs, including GB-derived neurosphere (NS) cell lines, show predominant CSPG4/ NG2 overexpression associated with stemness markers (nestin and vimentin, but not CD133) in tumour and perivascular cells [16, 92] (Fig. 5.3). GB patients with a high level of CSPG4/NG2 expression are characterised by shorter survival and increased resistance to radiotherapy [92]. NS cell lines show crosstalk between CSPG4/ NG2+ and CSPG4/NG2− cells, with the former proliferating faster and being more aggressive in vivo. Lethal GBs can also arise from CSPG4/ NG2– cells because a small minority may escape short hairpin (sh)RNA inhibition [91]. It is possible that CSPG4/NG2 acts as a sort of traffic light that modulates GB cell behaviour on the basis of environmental stimuli and favours programmed proliferation (CSPG4/NG2+ cells) or migration/invasion (CSPG4/NG2− cells) [91]. The expression of CSGP4/NG2 on reactive astrocytes in GBs may contribute to variable and focal positivity in the tumour parenchyma [94]

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Fig. 5.3  CSPG4/NG2 immunohistochemistry. (a) WHO grade II astrocytoma. Negative tumour cells and CSPG4/ NG2-positive reactive astrocytes; DAB, ×400. (b) IDH-­ wild type glioblastoma (GB). CSPG4/NG2-positive area; DAB, ×200. (c) Id. Diffuse CSPG4/NG2 staining on cell membranes; DAB, ×200. (d) WHO grade III oligodendroglioma. CSPG4/NG2-positive area with honeycomb appearance; DAB, ×400. (e) Id. Alcian Blue staining; ×200. (f) Id. CSPG4/NG2-negative tumour cells and -positive endothelial cells; DAB, ×200. (g) Id. Isolated CSPG4/NG2-positive tumour cells in infiltration area; DAB, ×400. (h) Id. CSPG4/NG2 expression in tumour

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cells; green immunofluorescence (IF), ×400. (i) IDH-wild type GB. Strong CSPG4/NG2 expression in vascular pericytes; DAB, ×200. (j) Id. CSPG4/NG2-positive vascular pericytes in glomerulus with sprouting; DAB, ×200. (k) Id. Negative tumour cells and CSPG4/NG2-positive reactive astrocytes in infiltration; DAB, ×200. (l) Id. CSPG4/ NG2-positive reactive astrocytes; DAB, ×200. (m) Id. ATRX+/GFAP+ reactive astrocytes, double staining; DAB/Fast RED, respectively, ×200. (n) Id. GB-derived cell lines, neurospheres. Most cells (but not all) are variably positive for CSPG4/NG2; green IF, ×200. (o) GB-derived cell lines, adherent cells are weakly positive for CSPG4/NG2; green IF, ×200 [127]

5  Chondroitin Sulphate Proteoglycans in the Tumour Microenvironment

(Fig.  5.3). This is in line with their origin from CSPG4/NG2+ and PDGFRα+ glial-restricted progenitors during neurogenesis and the dismal significance associated with reactive astrocytes [94]. Tumour-associated reactive astrocytes interact with tumour cells and trigger: (i) proliferation through the CXCL12 (SDF-1)/CXCR4 axis; (ii) migration and invasiveness by activating MMP-2; and (iii) survival by releasing different cytokines within the TME [95] (Fig. 5.1). In GB, CSPG4/NG2 can also be detected on the pericytes and ECs of proliferated tumour vessels (microvascular proliferations and glomeruli), as well as on microglia/macrophages [94] (Fig. 5.3). In addition to CSPG4/NG2, core proteins and biosynthetic enzymes are also predominantly overexpressed in human GBs [3]. CSs and GAGs were first biochemically and histochemically demonstrated in human gliomas [96–99] and rat tumours transplacentally induced using N-ethyl-N-nitrosourea (ENU) [100–102]. It was reported that CS was variably distributed on vessel walls and the cytoplasmic membranes of tumour cells mainly in regressive events and inversely correlated with dedifferentiation. Alcian blue positivity for CS is detected in isomorphic ENU oligodendrogliomas and in the peripheral part of polymorphic gliomas [101, 103].

5.4.2 Mechanisms of Signal Transduction by CSPG4/NG2 in Gliomas CSPGs can regulate multiple steps of the tumorigenesis of human gliomas: (i) CSPG4/NG2 interacts extracellularly through its core protein with collagen types II, V and VI, or laminin and tenascin [104], which are involved in the invasive behaviour of glial tumour cells [105]. (ii) CSPG4/NG2 acts as a co-receptor for spreading and focal contact in association with β1 integrin, a critical molecule in tumour cell adhesion and migration [106].

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(iii) β1 integrin is the most abundant integrin subunit in gliomas and favours invasiveness [107]. (iv) Actin (the intracellular binding partner of CSPG4/NG2) triggers cell motility and development [50]. (v) CSPG4/NG2 enhances the activity of PDGF-AA, a well-known mitogen for in vitro glioma cell growth [108], and its α receptor signalling pathway in SM and O2A cells [60, 109]. (vi) CSPG4/NG2 promotes α3β1 integrin activation by triggering the downstream activation of FAK and PI3K/AKT signalling and increasing glioma cell proliferation, motility and survival (Fig. 5.1) [48]. (vii) CSPG4/NG2 regulates the expression of intercellular adhesion molecule 1 (ICAM-­ 1), an essential protein for leukocyte adhesion and transmigration in pericytes and GB tumour cells [110].

5.4.3 CSPG4/NG2 in Blood Vessel Development The progression of solid tumours is triggered by factors that are intrinsic to tumour cells and by extrinsic factors present in the TME. The effects of stromal factors allow tumour cells to spread to distant sites, whereas the interactions of tumour cells and components of the TME support tumour growth, progression and neo-angiogenesis. CSPG4/NG2 is involved in blood vessel development under normal and pathological conditions, including tumours. It is expressed in the vasculogenic and angiogenic neo-vasculature in which crucial crosstalk between ECs and mural cells is mediated by various growth factors, including PDGFR-AA and -BB [111]. The latter intervenes at different times during vascularisation and affects ECs and mural cells in the developing neo-vasculature. A tube formation has been described in the absence of ECs but in the presence of CSPG4/NG2− and PDGFRα-­ expressing cells [112]. The surfaces of perivascular cells (including ECs in normal brain vessels [112, 113] and peri-

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cytes of proliferated tumour vessels in malignant gliomas) express CSPG4/NG2 [114, 115] (Fig. 5.3), which is upregulated in pericytes of the neo-vasculature, downregulated in quiescent vasculature and absent or undetectable in stable healthy human adult brain vessels [116]. Mice lacking CSPG4/NG2 have defective vasculature [117]. CSPG4/NG2 and PDGFRβR, which is responsible for the ability of pericytes to respond to PDGFR-BB [118], are the most reliable markers of activated pericytes [112]. Their combined use allows their contributions to neo-vascularisation to be detected. Vascular hyperplasia, which involves the aggressive recruitment of pericytes and ECs, is a distinctive feature of malignant gliomas, particularly GB (Fig. 5.3). Pericyte subsets show differential CSPG4/NG2 expression in GBs [119], and only one specific type of pericyte expressing both CSPG4/NG2 and nestin is thought to be recruited during tumour angiogenesis [120]. In the developing human brain and in brain tumours, NG2/CSPG4 favours the motility and angiogenesis of perivascular cells by means of galectin-3 and α3β1 integrin [45] (Fig.  5.1). Reduced pericyte interactions with ECs in PC-NG2ko mice leads to the loss of the pericytic activation of β1 integrin signalling in ECs, whereas reduced pericyte-EC interactions in Mac-NG2ko mice reduces macrophage recruitment by 90% [52]. Most of the pericytes in GB derive from GB stem cells (GSCs), and their selective deprivation disrupts the neo-vasculature and inhibits tumour growth. GSCs reside in the perivascular niche (PVN), are supported by trophic factors from the vasculature and may undergo mesenchymal differentiation giving rise to pericytes [121, 122]. They are recruited towards ECs through the CXCL12 (SDF-1)/CXCR4 axis and are induced to become pericytes by transforming growth factor β (TGF-β) [123]. GSCs may also contribute to perivascular pericytes by actively remodelling the PVN [123–126]. Figure 5.3 shows personal findings concerning CSPG4/NG2 immunoreactivity in human gliomas and GB-derived cell lines [127].

5.4.4 CSPG4/NG2 in Non-glial Tumours CSPG4/NG2 plays a role in the progression of multiple tumour types other than gliomas. CSPG4/ NG2 overexpression in radial growth phase melanomas triggers migration, protease activation, and epithelial to mesenchymal transition (EMT), favouring progression from a radial to vertical growth phase phenotype [28]. It is also detected in subsets of childhood acute lymphoblastic and acute myeloid leukaemia where is associated with a worse prognosis in patients harbouring 11q23 translocations [128]. Aberrant CSPG4/NG2 expression has recently been found in renal and pancreatic cell carcinomas, chondrosarcomas, osteosarcomas, triple-negative breast cancer and squamous carcinomas of the head and neck [5, 51].

5.5

CSPG4/NG2 as a Therapeutic Target

5.5.1 CSPG4/NG2 in the Treatment of Gliomas In gliomas, CSPG4/NG2 plays a central role in tumour cell proliferation through PDGFRα and FGF-2. Its aberrant overexpression in 50–67% of GBs, together with PDGFRα and Olig2, supports its use as a potential therapeutic target [7]. Consequently, the possibility of exploiting the theranostic properties of CSPG4/NG2 in gliomas and other tumour types has aroused considerable interest [5, 10]. CSPG4/NG2 overexpression is also positively associated with multidrug resistance, which is mediated by the increased activation of α3β1 integrin, PI3K/Akt signalling and their downstream targets, promoting cell survival [92, 129]. CSPG4/NG2 knockdown with shRNAs incorporated into lentiviral vectors attenuates β1 integrin signalling and has potent antitumour effects by sensitising tumour cells to cytotoxic treatment in vitro and in vivo [129]. In xenografts of the U87-MG GB cell line in athymic nude mice, chemoimmunoconjugates of

5  Chondroitin Sulphate Proteoglycans in the Tumour Microenvironment

anti-CSPG4/NG2 mAb9.2.27 and vinblastine lead to long-term growth suppression [114]. A similar effect on tumour growth and angiogenesis has also been obtained in xenografts of GB-derived cell lines overexpressing CSPG4/ NG2 by means of the intra-cerebral delivery of lentivirally encoded shRNAs [9]. Targeting CSPG4/NG2 with mAb9.2.27 and activated natural killer cells inhibits tumour growth and improves the survival of GB-bearing animals by favouring a pro-inflammatory TME [11, 12], as in a rat model of GB [13]. In comparison with single epitope targeting, a significant reduction in GB cell viability has been successfully obtained using a Mab-Zap saporin immunotoxin system to ablate CSPG4/NG2 and GD3(A), a ganglioside expressed by developing migratory glia [14]. The Cre-lox method for the cell type-specific ablation of CSPG4/NG2 [26] impairs tumour vascularisation in intracranial implantations of B16F10 melanoma cells in mice as a result of the loss of the CSPG4/NG2-mediated activation of β1 integrin signalling in pericytes [130].

5.5.2 CSPG4/NG2 as a Target for Immunotherapy Given its overexpression in tumour cells, CSPG4/ NG2 is an attractive candidate for antibody-based therapeutic approaches to solid tumours, including the use of specific anti-CSPG4/NG2 antibodies and immuno-based therapies (e.g. chimeric antigen receptor T [CAR-T] cell therapy) [15, 131–133]. As anti-CSPG4/NG2 mAbs inhibit tumour progression by blocking ligand access to extracellular CSPG4/NG2 binding sites, CSPG4/ NG2-directed antibody conjugates are selectively internalised by CSPG4/NG2-expressing tumour cells as a result of endocytosis [132]. The upregulation of CSPG4/NG2  in tumourassociated pericytes means that this approach could contribute to tumour regression by inhibiting neo-­ angiogenesis in malignant brain tumours [15, 131].

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The development of immunotherapy has led to significant progress in the treatment of indolent and metastatic tumours. By means of genetic engineering technologies, T lymphocytes can be redirected to recognise and target a wide variety of tumour-associated antigens (TAAs) through the expression of CAR-Ts. CARs are hybrid proteins in which the binding moiety derived from a mAb is fused with a signalling molecule of the CD3/T cell receptor complex and co-stimulatory endodomains. In order to overcome the need for T cells to recognise the TAAs presented by the major histocompatibility complex (MHC), CAR-T cells are genetically modified to express a chimeric T cell receptor that recognises the antigen of interest and redirects cytotoxic T cells to tumour cells. When inserted into T cells, CARs confer MHC-independent cytotoxic activity and promote T cell proliferation, activation and persistence both in vivo and in vitro [134]. Clinical trials of CAR-transduced peripheral blood lymphocytes have successfully led to the remission of both solid and haematological malignancies. In particular, redirected T cells expressing a CSPG4/NG2-specific CAR may be a promising new means of targeting a wide range of indolent solid tumours including GB [131, 135]. In a preclinical study, anti-CSPG4/NG2 CAR-T cells successfully induced growth arrest in GB-derived NS and glioma xenograft models, without any signs of immune evasion [16]. Remarkably, anti-CSPG4/NG2 CAR-T therapy is also effective in GB-derived NS expressing moderate to low CSPG4/NG2 levels, an effect mediated by the in vivo upregulation of CSPG4/ NG2 under conditions of inflammation and hypoxia [136]. Inflammatory cytokines, including interleukin-1β (IL-1β) and tumour necrosis factor α (TNF-α), induce CSPG4/NG2 expression on tumour cells and microglia/macrophages, favouring their extravasation into the tumour mass. In their turn, antigen-activated CAR-T cells produce TNF-α in the glioma TME and induce CSPG4/NG2 expression even in the CSPG4/NG2– fraction of GB cells [137]. Constitutive and TNF-α-inducible CSPG4/NG2 expression may reduce the risk of tumour cell

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escape when targeted antigens are heterogeneously expressed on tumour cells. At the same time, it keeps the tumour susceptible to targeted therapy, including CAR-T cell or mAb therapy (mAb9.2.27) [15, 138]. Two particular phenotypic features of GB must be considered when using this therapeutic strategy: (i) the presence of circumscribed necrosis with GSCs/progenitors spared from the advancing necrotic process or induced by TME/ necrosis [139]; (ii) OPC and macrophage/microglia proliferation at the tumour border favours the tumour cell acquisition of a stem cell profile and chemoresistance. Known as the “border niche”, this sanctuary may be very important when considering therapeutic strategies [122, 124–126].

CAR-T cell therapy, and immunotherapeutic anti-CSPG4/NG2 CAR-T cell therapy may also be effective for malignant brain tumours because of the possibility of overcoming tumour escape and intra-tumour heterogeneity. Increasing our knowledge of the role of CSPG4/NG2  in oncogenic signalling and TME interactions in GB (including the switch from proliferative to invasive programmes) will be critical objectives of investigations aimed at discovering all of the possible theranostic implications of this macromolecule.FundingThis work was supported by Fondazione Compagnia di San Paolo (Turin, Italy) (Grant No. 2016.AAI2705. U3302) and Fondazione Edo ed Elvo Tempia Valenta – ONLUS (Biella, Italy).

5.6

References

Conclusions

Over the last few decades, a number of studies have shown that CSPG4/NG2 is a key factor in the development of the CNS and neuronal function, as well as in experimental and human glial tumours. Its role in CNS development, neo-­ angiogenesis and gliomagenesis emphasises its potential as therapeutic target, and new insights suggest that this may contribute to the defeat of glial neoplasia in the near future. Studies of the function of CSPG4/NG2 have proved to be very useful in furthering our understanding of CNS biology, mainly because of its involvement in the cytogenesis of neurons or glia cells and normal neo-vasculature. They have also been important in clarifying the origin of gliomas and improving prognostic predictions. The dynamic expression of CSPG4/NG2 during cytogenesis could be used to establish the timing of the malignant transformation of gliomas and its significance in each molecular subtype. It would also be very interesting to verify its possible relationships with stemness or differentiation markers. The increased expression of CSPG4/NG2  in many malignancies has led to it being considered a prototype oncoantigen and a putative candidate for targeted treatments. Clinical studies of malignant melanomas and triple-negative breast cancer have shown that it is a promising target for

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5  Chondroitin Sulphate Proteoglycans in the Tumour Microenvironment 105. Pilkington GJ (1996) The role of the extracellular matrix in neoplastic glial invasion of the nervous system. Braz J Med Biol Res 29(9):1159–1172 106. Iida J, Meijne AM, Spiro RC, Roos E, Furcht LT, McCarthy JB (1995) Spreading and focal contact formation of human melanoma cells in response to the stimulation of both melanoma-associated proteoglycan (NG2) and alpha 4 beta 1 integrin. Cancer Res 55(10):2177–2185 107. Pilkington GJ (1994) Tumour cell migration in the central nervous system. Brain Pathol 4(2):157–166 108. Kirsch M, Wilson JC, Black P (1997) Platelet-­ derived growth factor in human brain tumors. J Neuro-Oncol 35(3):289–301 109. Grako KA, Stallcup WB (1995) Participation of the NG2 proteoglycan in rat aortic smooth muscle cell responses to platelet-derived growth factor. Exp Cell Res 221(1):231–240 110. Schmitt BM, Laschke MW, Rössler OG, Huang W, Scheller A, Menger MD, Ampofo E (2018) Nerve/ glial antigen (NG) 2 is a crucial regulator of intercellular adhesion molecule (ICAM)-1 expression. Biochim Biophys Acta, Mol Cell Res 1865(1):57– 66. https://doi.org/10.1016/j.bbamcr.2017.09.019 111. Ozerdem U, Grako KA, Dahlin-Huppe K, Monosov E, Stallcup WB (2001) NG2 proteoglycan is expressed exclusively by mural cells during vascular morphogenesis. Dev Dyn 222(2):218–227 112. Ozerdem U, Stallcup WB (2003) Early contribution of pericytes to angiogenic sprouting and tube formation. Angiogenesis 6(3):241–249 113. Pouly S, Prat A, Blain M, Olivier A, Antel J (2001) NG2 immunoreactivity on human brain endothelial cells. Acta Neuropathol 102(4):313–320 114. Schrappe M, Klier FG, Spiro RC, Waltz TA, Reisfeld RA, Gladson CL (1991) Correlation of chondroitin sulphate proteoglycan expression on proliferating brain capillary endothelial cells with the malignant phenotype of astroglial cells. Cancer Res 51(18):4986–4993 115. Wesseling P, Schlingemann RO, Rietveld FJ, Link M, Burger PC, Ruiter DJ (1995) Early and extensive contribution of pericytes/vascular smooth muscle cells to microvascular proliferation in glioblastoma multiforme: an immune-light and immune-­electron microscopic study. J Neuropathol Exp Neurol 54(3):304–310 116. Virgintino D, Girolamo F, Errede M, Capobianco C, Robertson D, Stallcup WB, Perris R, Roncali L (2007) An intimate interplay between precocious, migrating pericytes and endothelial cells governs human fetal brain angiogenesis. Angiogenesis 10(1):35–45 117. Huang FJ, You WK, Bonaldo P, Seyfried TN, Pasquale EB, Stallcup WB (2010) Pericyte deficiencies lead to aberrant tumor vascularizaton in the brain of the NG2 null mouse. Dev Biol 344(2):1035– 1046. https://doi.org/10.1016/j.ydbio.2010.06.023

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6

Lipoproteins and the Tumor Microenvironment Akpedje Serena Dossou, Nirupama Sabnis, Bhavani Nagarajan, Ezek Mathew, Rafal Fudala, and Andras G. Lacko

Abstract

The tumor microenvironment (TME) plays a key role in enhancing the growth of malignant tumors and thus contributing to “aggressive phenotypes,” supporting sustained tumor growth and metastasis. The precise interplay between the numerous components of the TME that contribute to the emergence of these aggressive phenotypes is yet to be elucidated A. S. Dossou · N. Sabnis · B. Nagarajan E. Mathew Lipoprotein Drug Delivery Research Laboratory, Department of Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, TX, USA R. Fudala Lipoprotein Drug Delivery Research Laboratory, Department of Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, TX, USA Department of Microbiology, Immunology and Genetics, University of North Texas Health Science Center, Fort Worth, TX, USA A. G. Lacko (*) Lipoprotein Drug Delivery Research Laboratory, Department of Physiology and Anatomy, University of North Texas Health Science Center, Fort Worth, TX, USA Departments of Physiology/Anatomy and Pediatrics, University of North Texas Health Science Center, Fort Worth, TX, USA e-mail: [email protected]

and currently under intense investigation. The purpose of this article is to identify specific role(s) for lipoproteins as part of these processes that facilitate (or oppose) malignant growth as they interact with specific components of the TME during tumor development and treatment. Because of the scarcity of literature reports regarding the interaction of lipoproteins with the components of the tumor microenvironment, we were compelled to explore topics that were only tangentially related to this topic, to ensure that we have not missed any important concepts. Keywords

Tumor environment · Lipoproteins Endothelial cells · Pericytes · Macrophages Exosomes · Fibroblasts · Neutrophils Microvasculature · Macrophage polarization Tumor aggressiveness · Metastasis · Immune surveillance · Adipose cells · High-density lipoproteins

6.1

Lipoproteins

Lipoproteins are pseudo-micellar structures, designed to transport essential water-insoluble molecules of dietary or metabolic origin: triacylglycerols for energy utilization and storage and cholesterol (Fig. 6.1) for membrane and hormone

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 A. Birbrair (ed.), Tumor Microenvironment, Advances in Experimental Medicine and Biology 1272, https://doi.org/10.1007/978-3-030-48457-6_6

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biogenesis and for bile acid synthesis. Lipoproteins, especially high-density lipoproteins (HDLs), also transport a broad range of substances, including drugs and nucleic acids [1], that could contribute to carcinogenesis, metastasis, or other (therapeutic) processes opposing these phenomena. As a result, lipoprotein-type delivery vehicles have been investigated as anticancer agents for decades [2], primarily because of their ability to specifically target malignant cells and tumors via receptor-mediated interactions [3]. The special affinity of the apolipoprotein components of the lipoprotein complexes or their mimetic surrogates [4] for specific receptors is likely to extend to macrophage receptors or to other surface antigens found in the

A. S. Dossou et al.

TME.  Consequently, lipoprotein-type carriers could become important therapeutic tools in the treatment of especially aggressive cancers. In addition, the study of the interaction between lipoproteins and components of the TME could reveal important information, inducing new therapeutic approaches for difficult to treat malignant tumors. Currently, information available regarding the role that lipoproteins play in cancer development, diagnostics, and therapeutics by interacting with components of the TME is very limited the scientific/medical literature. This review is an attempt to focus attention on these interactions that will likely play a significant role in the design and development of novel diagnostic and treatment approaches in oncology.

Fig. 6.1  Schematic structure of a generic plasma lipoprotein (From: Bioscience notes. Online biological notes for students http://www.biosciencenotes.com/lipoproteins-introduction-structure-and-function/)

6  Lipoproteins and the Tumor Microenvironment

6.2

Contributions of Specific Components of the TME to Tumorigenesis and Metastasis

Cancer has been recognized as a highly heterogeneous disease, involving a multicomponent tumor environment that is comprised a variety of resident and infiltrating host cells including myofibroblasts, neuroendocrine cells, adipose tissue, immune inflammatory cells, blood, and other

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vascular lymphoid networks [6], in addition to tumor cells (Fig. 6.2). The interplay between these cells provides growth signals, secretory factors, metabolites, and extracellular matrix proteins and thus creates a favorable environment, collectively known as the stroma, for tumor growth and metastasis. A deeper understanding of the interactions within the TME has now revealed additional details, including key secretions originating from tumor cells that have the ability to alter the phenotypes

Fig. 6.2  Ingredients frequently found to contribute to interactions within the tumor environment (From: Foster et al. [5])

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of immune cells and thereby suppressing the immune system and thus promote tumor growth and metastatic processes [7, 8]. It has recently been shown that tumor diversity is enhanced via the varying components of their environment, including stromal cell proportions or activation states. In response to evolving environmental conditions and oncogenic signals from growing tumors, the TME continually changes over the course of cancer progression, underscoring the importance of the TME and the understanding of how tumor cells influence the assembly of their environment [9]. Based on the current findings of Wang et al. [6], the TME is highly influenced by 10 important characteristics including: (a) Uncontrolled multiplication (b) Escape from growth suppressors (c) Promotion of invasion and metastasis (d) Resisting apoptosis (e) Stimulating angiogenesis (f) Maintaining proliferative signaling (g) Elimination of cell energy limitation (h) Evading immune destruction (i) Genome instability and mutation (j) Tumor-enhanced inflammation Besides these factors that contribute to an optimal environment for tumor growth, other conditions, including acidity (low pH) in the TME, may also be important contributors to angiogenesis and metastasis [10]. Additionally, a bidirectional communication between the tumor cells and the microenvironment is another critical factor that influences the initiation and progression of malignancy and eventually patient prognosis [11]. Hence, understanding the role of the tumor microenvironment and monitoring its changes via molecular and cellular profiles during tumor progression could be vital for identifying cellular or protein targets for cancer prevention and therapy [12, 13]. As described above, tumorigenesis is a complex three stage and dynamic process: initiation, progression, and metastasis. The tumor microenvironment (TME) consisting of stromal cells and extracellular matrix (ECM) has been shown to be modified (educated) by tumor secretions, leading to a synergistic relationship between the constitu-

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ents of the TME and the tumor itself. Consequently, the steps involved in tumorigenesis are determined/influenced by the composition and functional activity of the TME, especially via immunogenic and non-immune cells. In the following sections, the opportunities for novel diagnostic and treatment approaches of malignant tumors will be discussed with a special focus on the potential interactions that may exist between lipoproteins and the TME.

6.2.1 Contributions of Lipids/ Lipoproteins to Tumor Acidosis in the TME Although a complex milieu in TME triggers oncogenic events that drive malignant developments; stress factors such as hypoxia and acidosis also have a key role in early tumor development. Many of the early metastatic events are supported by a “reversed pH gradient,” representing fluctuations between intracellular and extracellular pH [14, 15]. Lipid/lipoprotein metabolism has been linked to changes in lipid availability, thus impacting the metastatic potential of malignant cells and tumors. Lipids accumulate in special cytoplasmic organelles, termed “lipid droplets” (LDs) or adiposomes, composed primarily of neutral lipids (triacylglycerols and cholesteryl esters) that primarily originate from lipoproteins [16, 17]. Lipid droplets also serve as a reservoir for cholesterol and acylglycerols for membrane biogenesis and energy-yielding metabolites [18, 19]. Several studies have shown abnormal accumulation of LDs in malignant tumors [17, 20– 23]. Koizume et al. [24] reviewed the production and functions of LDs in cancer cells in relation to the cellular environment, including tissue oxygenation status and metabolic activity. These findings contributed to the current understanding of how cancer cells adapt to diverse tumor environments to promote their survival via accelerating their lipid metabolism. In another study, LD accumulation has been demonstrated to be affected by environmental stresses, including acidosis, hypoxia, as well as chemoresistance [22] in addition to de novo lipogenesis and extracel-

6  Lipoproteins and the Tumor Microenvironment

lular lipid uptake [25, 26]. In the TME, LDs may also play an important role in ER homeostasis [27] and as ROS scavengers [28, 29].

6.2.2 Cancer-Activated Fibroblasts (CAFs) Fibroblasts are cells that regulate the structure and function of healthy tissues through the formation of the extracellular matrix (ECM). Myofibroblasts, which are activated fibroblasts, are transiently present at the site of tissue injury, involved in wound repair, and progressively disappear via an apoptotic mechanism. However, during cancerous tissue fibrosis, myofibroblasts, called cancer-associated fibroblasts (CAF), are permanently activated and are essential for initiating and promoting tumor growth [30–32]. CAFs are one of the most dominant components in the tumor stroma [33] and have been identified to exert a significant control on the rate and the progression of tumor growth by remodeling ECM proteins, secreting growth factors and immunosuppressive cytokines, recruiting inflammatory cells, inducing angiogenesis, and enhancing cancer cell proliferation via mesenchymal-epithelial cell interactions. In addition, there are several subsets of CAFs that have been known to exhibit specialized functions in the carcinogenesis. Investigation of proliferative ability of CAFs has revealed that α-smooth muscle actin (α-SMA)-expressing CAFs (also known as myofibroblasts) promote the proliferation of cancer stem cells by utilizing the interaction between the chemokine receptor and its ligands, CXCR4 and CXCL12 [34]. A specific subset of CAFs in pancreatic cancer (that express α-SMA, vimentin, and glial fibrillary acidic protein) secrete macrophage colony-stimulating factor (M-CSF), interleukin 6 (IL-6), and CC-chemokine ligand 2 (CCL2) and thus promote monocyte recruitment and additionally macrophage differentiation and polarization to the M2 (tumor-promoting) tumor-associated macrophages (TAM) phenotype [35, 36]. The α-SMA-secreting CAFs also secrete cytokines (IL-6, IL-8, TGF-, and IL-10)

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to recruit monocytes and to promote their differentiation toward the M2 phenotype. Interestingly, M2 macrophages further activate the CAFs leading to further (self-catalytic) tumor progression [37, 38]. The expression of M2 macrophage markers (CD163 and DC-SIGN) and CAF markers (α-SMA, S100A4, and FAP (fibroblast-activating protein)) has been predictors of poor clinical outcomes in squamous cell carcinoma and colorectal cancer patients [39, 40]. A potential therapeutic approach involving lipoprotein transport that is discussed in more detail below, under the topic dealing with TAMs, could potentially also be useful against CAFs via limiting the polarization of TAMs toward the tumor-promoting M2 phenotype. Takai et al. [41] reported substantial reduction of tumor growth in triple-negative breast cancer xenografts of patient-derived tumors in mice using pirfenidone, a lipophilic anti-CAF agent. Utilizing lipoprotein-based drug delivery that is exceptionally suitable for transporting lipophilic drugs, in addition to the selective targeting potential via the scavenger receptor class B type 1 (SR-B1) receptor [16], could significantly enhance this therapeutic approach.

6.2.3 Neuroendocrine Cells Neuroendocrine tumors (NETs) are heterogeneous malignancies that arise in specialized neuroendocrine (NE) cells. These cells are part of the diffuse neuroendocrine system which exhibits a combination of both neuronal and endocrine features [42]. In general, NE cells respond to neurological signals by secreting and releasing hormones and peptides into the bloodstream, inducing physiological and local regulatory functions in specific organ sites, including the regulation of appetite, body temperature, pH, etc. [43]. NETs may be functioning or nonfunctioning tumors as exemplified by pancreatic tumors that produce a variety of peptide hormones including insulin, glucagon, and gastrin, whereas metastatic NETs have been found to secrete serotonin and other vasoactive substances [44].

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Recently, tumor neoangiogenesis has been observed to play the key role in NET progression. The TME makes an important contribution to the progression of NETs and to the pathogenesis of fibrotic complications of carcinoid heart disease and mesenteric desmoplasia. NE cells also play an important role in the development of NE tumors. Accordingly, investigation of NE cells in prostate carcinoma has revealed that their secretory products enhance the proliferation of prostate tumors (by inhibiting apoptosis and stimulating neoangiogenesis) and thus have been linked to tumor aggressiveness [45]. Although rare, the incidence of NETs is relatively high in the gastroenteropancreatic (GEP) tract and in the bronchopulmonary (BP) tract [44]. Recent studies show that both gastrointestinal NETs and pancreatic NETs respond to everolimus and temozolomide [46]. We have recently shown that both of these agents were effective in suppressing the growth of glioblastoma cells, especially when delivered via high-density lipoprotein-type drug transport system to SR-B1 overexpressing cells [47]. A similar drug delivery approach might be effective against NET cells if the reconstituted high-density lipoprotein nanoparticles (rHDLs) are augmented by conjugation of somatostatin mimetic peptides, targeting NET surface receptors [48].

6.2.4 Neutrophils Originating from the bone marrow, neutrophils are short-lived cells and represent the most abundant circulating leukocytes, and along with macrophages, they make up the first line of defense of the innate immune system. Recruited from the circulation by cytokines and chemokines, neutrophils infiltrate tissues and work to contain infection. Phagocytosis and subsequent degradation of the engulfed microbes via proteases and respiratory burst constitute a major mechanism of the antimicrobial activity of neutrophils [49, 50]. Another method that neutrophils utilize to target microbes is degranulation in the extracellular space. Diverse types of granules have been identified in neutrophils, including azurophil granules

A. S. Dossou et al.

that take up the azure A dye and store the microbicidal azurocidin and myeloperoxidase (MPO) [51]. Specific granules contain the non-heme iron-binding protein, lactotransferrin, that contributes to the antimicrobial effects [52–55]. Anti-inflammatory effects have also been reported for lactotransferrin while studying microbial and auto-immune diseases [56–58]. Besides actively fighting of microbes, neutrophils can also modulate the function of other immune cells including macrophages, dendritic cells, monocytes, natural killer (NK) cells, T cells, and B cells via direct interaction or the release of cytokines [59]. Meta-analyses across several cancer types and stages show that high neutrophil-to-lymphocyte ratio in the blood and high intratumoral neutrophil infiltration correlate with adverse outcomes [60–62]. Despite their prognostic value, controversy surrounds the role of neutrophils in the TME, as they can release both cytotoxic, proangiogenic, and pro-metastatic factors [63]. For example, neutrophils from healthy donors exert cytolytic activity on cancer cells via the production of reactive oxygen species [64]. Paradoxically, tumor-associated neutrophils (TANs) represent a major source of metalloproteinase-9 (MMP-9) that can break down the extracellular matrix (ECM), inducing the release of VEGF from the ECM [65–67]. Studies in mice advanced the identification of two subsets of TANs: the proinflammatory, anti-tumoral N1, and the immunosuppressive, pro-tumoral N2 TANs with the N2 phenotype induced by TGF-β in the TME [68]. In line with these observations, high N2-to-N1 ratio positively correlated with lung cancer aggressiveness [69]. Moreover, Eruslanov et  al. reported that in early-stage lung cancer, “activated” (N1-like) TANs are able to stimulate T cell proliferation and activation [70]. Besides the N1 versus N2 functional model, other descriptions of pro-tumoral versus anti-tumoral neutrophils have been reported including the low-density neutrophils versus the normal-density neutrophils and the pro-tumoral immature granulocytic myeloidderived suppressor cells [71].

6  Lipoproteins and the Tumor Microenvironment

Lipoproteins and TANs  Earlier studies of vascular inflammatory diseases showed that lipoproteins do affect the function of neutrophils. In vitro treatment of neutrophils with Apo A-I and HDL or treating mice with Apo A-I and HDL or pre-treating patients with peripheral vascular disease with reconstituted high-density lipoproteins resulted in decreased neutrophil activation, adhesion, and migration. This dampening of neutrophil activation was attributed to the interaction of Apo A-I and HDL, respectively, with the ABCA1 and SR-B1 receptors [72]. Divergent effects on neutrophils have been described on native and oxidized LDL. In a model of chronic hypercholesterolemia with an elevated level of oxidized LDL, neutrophil calcium influx and chemotaxis were impaired, whereas expression of CD11b-surrogate for activation state of neutrophils was not altered. Although both native LDL and oxidized LDL significantly induced the expression of the monocyte chemoattractant protein 1, treatment with native LDL increased CD11b expression, chemotaxis, and calcium flux in neutrophils, whereas oxidized LDL abrogated these effects and promoted apoptosis of neutrophils [73]. Obama et al. showed that oxidized LDL, but not native LDL, enhanced phorbol 12-myristate 13-acetate-induced NET formation and myeloperoxidase production in neutrophils [74]. Because oxidized LDL is a contributor of carcinogenesis ([75]; [76]), investigation of the interaction between lipoproteins and infiltrating neutrophils and how these interactions may modulate the function of neutrophils in the TME is likely to lead to important advances and perhaps novel diagnostic and therapeutic approaches.

6.2.5 Endothelial Cells in the TME Cancer cells require a constant, vigorous supply of nutrients and oxygen to facilitate the rapid growth and the homeostasis of the TME. Angiogenesis, including the growth of new capillaries, is also stimulated by oxygen and nutrients. Apparently, during hypoxia or oxidative stress,

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tumor cells are under stress to secrete proangiogenic ligands that induce the endothelial cells to initiate the process of angiogenesis or neovascularization. The tumor-associated endothelial cells (TNECs) play two key roles in the TME: 1. TNECs enable the development of new blood vessels from the existing blood vessels or by recruiting bone marrow-derived progenitor endothelial cells. 2. TNECs mechanistically control the recruit ment of leukocytes, tumor cell behavior, and consequently even metastasis (Chouaib S) as they are crucial components of the interface between the circulating blood cells, tumor cells, and the extracellular matrix (ECM). Tumorigenesis may be impacted differently under hypoxia or inflammation [77]. For instance, HDL has been found to augment angiogenesis during hypoxia, while it has been reported to inhibit angiogenesis under inflammatory conditions. The precise mechanism involving the role of HDL and endothelial cells in the TME is still under investigation as Yu et al. [78] reported that endothelial lipase (LIPG) may be involved in HDL- induced angiogenesis. LIPG is a phospholipase, secreted by endothelial cells, and plays a vital role in lipoprotein metabolism, specifically in HDL metabolism. Accordingly, several studies attributed the effect of HDL on angiogenesis to its association with its active lipid ligand, sphingosine phosphate [79, 80]. HDL contains sphingosine-1-phosphate (S1P) that besides triggering a vascular response is also a substrate for S1P receptor (S1PR) on endothelial cells [81]. Subsequently, studies conducted by Tatematsu et  al. [82] invoked a possible mechanism for LIPG in tumor angiogenesis. Tatematsu’s observations indicated that LIPG-mediated hydrolysis of HDL releases and activates S1P which upon binding to the S1PR promotes the phosphorylation of protein kinase B and endothelial nitric oxide synthase [82]. Consequently, this leads to the migration of endothelial cells and hence angiogenesis. Besides playing a role in angiogenesis, LIPG has also been observed to contribute to cell growth, cell proliferation, and some of

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the crucial events in cancer progression [83]. Hence, LIPG could be explored as a potential target for cancer therapy. Intriguingly, several studies have also identified role(s) for apolipoprotein A-I (Apo A-I), the primary protein component of HDL, and its mimetic peptides in angiogenesis. Studies conducted in a mouse model of ovarian cancer indicated that the mice expressing the Apo A-I transgene had decreased tumor development and increased survival compared to the wild type mouse [84]. In another similar study, the effect of Apo A-I in tumorigenesis was investigated by analyzing the tumorassociated blood vessels in Apo A-I transgenic and knockout mice. The results indicated a significant decrease in the number of tumor blood vessels in addition to the reduction in vessel length, size, area, and density in the Apo A-I transgenic mice compared to the Apo A-I knockout mice [85]. Regarding the contribution of Apo A-I mimetic peptides to angiogenesis, subcutaneous or oral injection of mimetic peptide L-4F in CT26 mouse colon adenocarcinoma model showed a reduction in the proliferation and the viability of cancer cells. Subsequently, the peptide also decreased the tumor burden and the neovessel expression in tumorigenic BALB/c mice [86]. Similarly, investigation of the anti-tumorigenic effects of another Apo A-I mimetic peptide, L-5F, also revealed that the inhibition of vascular endothelial growth factor (VEGF)/basic fibroblast growth factor (bFGF)-mediated angiogenesis was partially responsible for the inhibition of tumorigenesis. In vitro studies showed that L-5F peptide inhibited the proliferation, invasion, migration, and the tube formation of endocytic cells by suppressing the VEGF/bFGF pathways [87]. Additionally, in vivo studies also indicated that the animals that received the peptide showed a decrease in the quantity and the size of the vessels in the tumor compared to the untreated group. Although several studies indicate that the anti-tumorigenic properties of HDL might occur via inhibition of angiogenesis, the contrasting characteristics of HDL with regard to hypoxia and inflammation suggest that its role in angiogenesis might be dependent on the type of cancer [77].

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6.2.6 Stroma and Endothelial Cells As increasing amounts of evidence indicate the presence of crosstalk between fibroblasts and NETs in the TME.  Both in  vitro and in  vivo experiments conducted to analyze the effect of CAFs on NETs indicate that CAFs could promote the growth of NETs [44, 88]. However, the proliferative ability of NET cells has been linked only to the α-SMA+ myofibroblasts and not to resting fibroblasts. Additionally, studies by Bowden et  al. [89] indicated that IL-6, VEGF, and monocyte chemoattractant protein 1 might play a role in stimulating NET cell proliferation based on comparing the secretomes of CAFs and normal human fibroblasts. NETs are one of the most highly vascularized cancers, as they overexpress a large number of proangiogenic factors, including VEGF, FGF, PDGF, semaphorins, and angiopoietins [90]. Consequently, NETs have a high intratumoral density, almost tenfold higher than other carcinomas [91]. Interestingly, the intratumoral microvascular density is higher in low-grade pNET tumors than in high-grade tumors, and it is also associated with improved prognosis and longer survival [92, 93]. Among the angiogenic factors expressed in pNETS, VEGF is the most effective promoting angiogenesis, as it is highly expressed in almost 80% of the NETS. Additionally, tumor expression of VEGF is higher in well-differentiated tumors compared to the poorly differentiated ones. VEGF and angiopoietins have been described as major contributors to NET progression, in agreement with the Durkin et al. [94] who found that angiopoietin is significantly upregulated in pNETs. Furthermore, in  vivo studies conducted by Rigamonti et al. [95] have shown that angiopoietin can increase the microvascular density of pNETs. Regarding the importance of the penetration of the endothelial and stromal barrier of the TME, Wilhelm et  al. [96] conducted groundbreaking studies, assessing the effectiveness of the delivery of anti-cancer agents to tumors and tumor cells via nanocarriers. After examining the findings from 232 datasets in 117 carefully selected

6  Lipoproteins and the Tumor Microenvironment

studies, they found that on the average, only 0.7% of the injected dose of nanoparticles actually reached the tumor. Follow-up studies [97] were even more alarming. These investigators specifically investigated the efficiency of tumor-targeted nanoparticles and found that only 0.0014% of the injected drug load actually reached the cancer cells within the tumor. The rest of the payload was apparently trapped in the TME. These findings brought into focus the significance of the physical/chemical and biological properties of the respective nanoparticles that have been used to deliver anti-cancer agents. In our hands, the reconstituted high-density lipoprotein nanoparticles (rHDL NPs) are highly efficient in eliciting nearly a 90% ovarian tumor suppression upon injecting only a small (5 μg) siRNA dosage [98]. Perhaps the extremely small size of the nanoparticles (88.2% of extravasated nanoparticles [97], indicating that these types of nanoparticles were 7–38 times more likely to interact with TAMs instead of cancer cells. Cell death was observed in both cancer cells (0.4%) and macrophage (3.6%) populations but more so in macrophage populations. When the nanoparticle dose was increased tenfold, there was an increase in the macrophage cell death (34.3%), but not in the cancer cells [97]. These findings led to speculation that the tumor shrinkage reported in other studies could be due to death of stromal cells and cancer cells indiscriminately rather than suppressing of tumor growth. Furthermore, it is evident that greater selectivity must be attained for nanoparticle delivery mechanisms, resulting in greater efficacy of therapy delivery by ensuring that the payload reaches the specific desired target [97]. A recent review [213] suggests that the types of tumors that are resistant to drug delivery (EPRinsensitive type) could be made accessible to che-

motherapy by treatments, including ultrasound and photodynamic therapy. As nanoparticles are likely to be considered by human tissues as foreign substances, the renal system and macrophages, associated with lymphoid organs or tumors, are likely to be involved in their clearance mechanisms. The macrophages found around cancer cells in the TME are termed tumor-associated macrophages (TAMs), and they play a particularly important role in the processing/disposal of nanoparticles [96]. Thus, to navigate the TME and reach its tumor target, the ideal nanoparticle would have to be small and uncharged and resist detection by macrophages as a foreign substance. Once it has passed these hurdles, it must either resist biological change or permutate in a manner that favors interaction with tumor cells. These criteria characterize lipoprotein-type drug carriers that have already proven to be highly effective during initial proof of concept studies [48] [98, 214]. Regarding the increased uptake (trapping) of nanoparticles by TAMs, their phagocytic properties could promote distribution of these drugtransporting nanoparticles away from cancer cells, their intended target [97]. Nanoparticles may be identified by macrophages as foreign substances and subsequently sequestered and degraded [96]. It has been observed that smaller nanoparticles are cleared at a slower rate than larger nanoparticles. Macrophages preferentially engulf particles with cationic charge, followed by those with anionic surface charge and finally by those with zero net surface charge. This observation has led to coating nanoparticles with neutral polymers as a form of camouflage. The capability of the nanoparticle to evade the macrophage is an important property, as longer half-life will allow an increased accumulation within the tumor. The physiochemical attributes of the nanoparticles are likely to change upon interaction with tissue constituents. In the design of drug-transporting nanoparticles, their properties should provide the appropriate features so that the resulting nanoparticle, subsequent to tissue interactions, is still capable of targeting tumor cells [96].

6  Lipoproteins and the Tumor Microenvironment

6.3.2 Lipoprotein TME Interactions Certain characteristics of the TME enhance cancer cell viability, especially when challenged by anti-cancer treatments. It has been postulated that components of the TME could be significant contributors to drug resistance as well. Consequently, an ideal cancer therapeutic approach would have to include the ability to penetrate the TME and carry out its intended function in order to exert the desired therapeutic effect. Tumor cells create an environment of hypoxia and acidosis that has been correlated with increased treatment resistance and tumor aggressiveness. In hypoxic conditions, there are numerous cellular changes where lipoproteins could play key roles. In glioblastoma cell lines, hypoxia induced the uptake of all three classes of lipoproteins (HDL, LDL, and VLDL) over a wide range of concentrations, as tested by quantitative flow cytometry studies. The effect was observed to be indirect (induced mechanism), as uptake of all lipoprotein classes significantly increased when the cells were preincubated in an acidic environment. There was no additional effect of hypoxia observed on lipoprotein uptake stimulated by acidosis [26]. As lipoproteins enter the intracellular environment, via endocytosis or via other mechanisms, changes in receptor expression, brought about by hypoxia, could be an important determinant in carcinogenesis and metastasis. Lipoproteins and their payloads may be internalized via binding to a variety of cell surface receptors, including VLDLR, LDLR, LRP1, and SR-B1. When laser micro-dissected regions of glioblastoma patient tumors were compared on the basis of hypoxic vs normoxic regions, LDLR was not induced by hypoxia. However, hypoxic induction of VLDLR, SR-B1, and LRP1 mRNAs was observed, with SR-B1 mRNA having the highest induction (fivefold at 24 hours). Hypoxic induction of VLDLR and SR-B1 proteins was quantified using an immunoblotting experiment as well, indicating stress-induced lipid loading into cancer cells in conjunction with upregulated receptor expression [26]. The marked (stressinduced) elevation of SR-B1 receptor expression validates the utility of therapeutic anti-cancer agent delivery via rHDL nanoparticles [3].

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Uptake of drug-carrying nanoparticles by non-malignant tissues can be considered to be an off-target, undesired outcome of therapy. However, targeted uptake by tumors is the main goal of therapeutics especially in oncology. A new approach involves coating nanoparticles with macrophage membranes, to selectively target tumors in breast cancer and lung cancer [215]. Macrophage cell membrane has been used to decorate gold nanoparticles, allowing the nanoparticles to target the tumor and remain in circulation longer, as tested in vivo. Stability of the macrophage coating is of concern, as shedding of the exterior macrophage membrane coat may result in additional unintended off-target effects [216].

6.4

Conclusion

The amount of information currently available in the literature regarding TME/lipoprotein interactions is very limited. We attempted to review all relevant information that might lead to improved diagnostics or therapeutics of solid tumors, at times a speculative venture, in addition to the published reports dealing with the potential role(s) that lipoprotein may play in regulating events in the TME. Acknowledgments  The authors’ research is supported by the Rutledge Cancer Foundation, the Texas Alzheimer’s Research and Care Consortium, the Virginia Kincaid Charitable Trust, and Wheels for Wellness, Fort Worth TX.

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7

The Role of BEHAB/Brevican in the Tumor Microenvironment: Mediating Glioma Cell Invasion and Motility Kristin A. Giamanco and Russell T. Matthews

Abstract

Malignant gliomas are the most common tumors in the central nervous system (CNS) and, unfortunately, are also the most deadly. The lethal nature of malignant gliomas is due in large part to their unique and distinctive ability to invade the surrounding neural tissue. The invasive and dispersive nature of these tumors makes them particularly challenging to treat, and currently there are no effective therapies for malignant gliomas. The brain tumor microenvironment plays a particularly important role in mediating the invasiveness of gliomas, and, therefore, understanding its function is key to developing novel therapies to treat these deadly tumors. A defining aspect of the tumor microenvironment of gliomas is the unique composition of the extracellular matrix that enables tumors to overcome the typically inhibitory environment found in the CNS. One conspicuous component of the gli-

K. A. Giamanco Department of Biological and Environmental Sciences, Western Connecticut State University, Danbury, CT, USA e-mail: [email protected] R. T. Matthews (*) Department of Neuroscience and Physiology, SUNY Upstate Medical University, Syracuse, NY, USA e-mail: [email protected]

oma tumor microenvironment is the neural-­ specific ECM molecule, brain-enriched hyaluronan binding (BEHAB)/brevican (B/b). B/b is highly overexpressed in gliomas, and its expression in these tumors contributes importantly to the tumor invasiveness and aggressiveness. However, B/b is a complicated protein with multiple splice variants, cleavage products, and glycoforms that contribute to its complex functions in these tumors and provide unique targets for tumor therapy. Here we review the role of B/b in glioma tumor microenvironment and explore targeting of this protein for glioma therapy. Keywords

Proteoglycan · Glioma dispersion · Glycosylation · ADAMTS4 · MMP · Lectican · Chondroitin sulfate · Fibronectin · Glioma-­ initiating cells · EGF receptor · Hyaluronan · ECM · TME · BEHAB · Glycoform

7.1

Introduction

The tumor microenvironment is comprised of the cells that directly make up the tumor, neighboring normal/non-transformed cells, the extracellular matrix (ECM), and secreted molecules

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 A. Birbrair (ed.), Tumor Microenvironment, Advances in Experimental Medicine and Biology 1272, https://doi.org/10.1007/978-3-030-48457-6_7

117

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found within this space [5, 52, 140]. Importantly, interactions between these constituents define the molecular properties of the specific tumor [52]. High-grade gliomas are the most frequently detected and virulent form of intracranial tumors, but they are also commonly impervious to currently available treatments, including surgery as well as chemotherapy and radiation [125]. One of the primary reasons as to why gliomas are insusceptible to these therapies is due to the fact that gliomas are able to infiltrate surrounding tissues and, thus, are considered to be highly invasive [109, 126]. Many of the components within the tumor microenvironment support the dispersion and heightened motility of glioma cells. Specifically, gliomas exhibit aberrant expression patterns of cell adhesion molecules, ECM molecules, proteolytic enzymes that remodel the ECM, and growth factors [5, 140]. One such ECM molecule that is overexpressed in gliomas is the chondroitin sulfate proteoglycan (CSPG): brain-enriched hyaluronan-binding protein (BEHAB)/brevican (B/b) [42, 67]. This enhanced expression of B/b leads to an increase in the aggressiveness of the resulting tumors [35, 62, 83, 98, 134, 154]. It is important to note that that there are a number of B/b isoforms that are upregulated in glioma samples [133]. Mechanistically, B/b is cleaved by a disintegrin and metalloproteainase with thrombospondin motifs (ADAMTS)-4 [86], and this causes the abnormal adhesion of glioma cells to fibronectin, and the overall motility of the glioma cells is enhanced [62], thereby leading to tumor invasion. Moreover, as a result of an increase in the presence of B/b, fibronectin secretion is increased, as is the expression of a number of cell adhesion molecules [62]. Taken together, these molecular and structural changes to the tumor microenvironment favor glioma cell invasiveness and movement, which contributes to the resistance of gliomas to current therapeutics [109]. In this chapter we first focus on the function of B/b in normal/non-transformed cells and then delve into the molecular composition of the tumor microenvironment. Next, we dissect the specific role that B/b plays in promoting glioma

cell invasion and motility. Additionally, the mechanisms underlying these processes will be discussed and will shed further light onto why the current treatments are not more effective at targeting gliomas and why targeting B/b could be a new therapeutic strategy.

7.2

Brevican: Structure and Function

7.2.1 Structure B/b is a member of the lectican family of CSPGs along with versican, neurocan, and aggrecan [7, 113, 147]. In terms of structure, members of the lectican family are quite homologous. More specifically, these CSPGs contain a N-terminus, which is known as the hyaluronan (HA)-binding domain and link protein-like region, that mediates interactions between the lectican family members and HA (see Fig. 7.1, [104]). The binding of these CSPGs to HA is a key step in overall organization of the ECM. Within the C-terminus, there is an epidermal growth factor (EGF)-like domain that is characteristic of proteins found within the ECM, a lectin-like domain, and a complement regulatory protein-like domain [120]. It is through this lectin domain that the lecticans can bind tenascin-R, a glycoprotein present within the ECM.  Furthermore, this region of CSPGs can also bind to glycolipids found on the cell surface that have been sulfated, which promotes cell adhesion [89]. CSPGs consist of a core protein that is decorated with chondroitin sulfate (CS) sugar chains, which bind to the protein in the CS attachment region (Fig. 7.1). The amount of sugar units that can be added to the protein varies widely between members of the lectican family with B/b having 1–3 CS chains that adorn the protein core [7]. The core protein has been shown to hinder neurite outgrowth in cultured neuroblastoma cells [64], and the CS region of the CSPG has also been reported to inhibit overall growth and regeneration in the central nervous system (CNS) [21, 70]. A substantial body of work utilizing the bacterial enzyme, chondroitinase ABC (chABC), to

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Full-Length B\b ADAMTS 4/5 Cleavage

Alternative Splicing

GPI anchor

N-linked

O-linked

Differential Glycosylation

Hyaluronan binding domain Link protein-like domain

Chondroitin Sulfates

EGF-like domain Lectin-like domain Complement regulatory-like domain Fig. 7.1  BEHAB/brevican structure diversity, glycosylation, and cleavage. B/b is made as a both a secreted and GPI-linked isoform. It is routinely cleaved at a defined ADAMTS4/5 cleavage site leading to N-terminal and C-terminal fragments. In addition, work has shown that there is a lot of microheterogeneity in the glycosylation of

this protein. In gliomas all forms of B/b are upregulated with the N-terminal cleavage fragment perhaps being the most critical functionally. In addition there are glioma-­ specific glycoforms that are generated that may provide ideal targets for glioma therapy

digest CS chains supports these findings. In these studies, application of chABC resulted in recovery after spinal cord injury [14, 75, 136]. Additionally, administration of chABC has been reported to enhance axonal regeneration within the CNS in undamaged animals [31, 90]. Pizzorusso and colleagues used this enzyme to reopen the critical period and, thus, restore plasticity within the visual system of adult rats [105]. Taken together, both the core protein and the CS region of CSPGs inhibit growth and regeneration within the CNS and thus play a key role in restricting overall brain plasticity. Specifically, B/b can exist in a number of different ways: a glycosylphosphatidylinositol form

that is anchored to the plasma membrane [119, 120] and a form that is secreted right into the ECM [119], and additionally, B/b can be present as a glycosylated proteoglycan or as a core protein that is not glycosylated (Fig.  7.1, [146]). Interestingly, the form that is anchored to the plasma membrane was detected primarily in white matter tracts where axons are located as well glial cells that were classified as diffusely distributed throughout the brain. The form that is secreted into the ECM was highly expressed in the gray matter within the cerebral cortex, hippocampus, cerebellum, and particular thalamic nuclei [119]. Through Western blot analysis, in the adult rat brain, the full-length B/b protein

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runs at 145 kDa, but cleavage products have been described at 90 kDa and 50 kDa [145, 154]. Temporally, B/b expression is first detected on embryonic day 15 (E15). In all assayed regions of the CNS, the onset of B/b expression occurred after neurogenesis and instead was consistent with the generation of glial cells [66]. It has also been demonstrated that B/b expression is upregulated in response to injuries within the brain [41]. Following a stab wound to the adult rat brain, B/b was detected in regions of active gliosis [65], and, similarly, B/b expression within astrocytes was increased in response to lesions introduced into the entorhinal cortex in rats [127]. In that same vein, B/b mRNA was dramatically increased within the glial scar following cryo-injury in mice [64].

7.2.2 Function B/b is one of the molecular constituents of the perineuronal net (PNN) that is found within the CNS. PNNs surround the cell body and proximal neurites of particular populations of neurons within the CNS.  Typically, these cells are GABAergic interneurons, but they also can be found around excitatory cells. This structure serves to restrict plasticity by closing the critical period [9, 17, 23, 24, 48, 53, 58, 59, 124, 153]. Other work postulates that PNNs provide a buffering mechanism to preserve the balance of cationic charges within the extracellular milieu [17, 18, 54, 55]. Using B/b deficient mice, Bekku and colleagues revealed that B/b regulates the assembly of the proteoglycan, phosphacan, and tenascin-­R at Nodes of Ranvier within the CNS [8], which is likely critical in action potential propagation. Proper B/b expression is also needed to maintain normal speeds of synaptic transmission at the calyx of Held in the medial nucleus of the trapezoid body within the brainstem, where PNNs are found in abundance [13]. Studies performed using B/b knockout mice highlight a potential role for B/b in modulating long-term potentiation in the CA1 region of the hippocampus. Of particular interest, the mice lacking B/b

displayed less prominent PNNs, meaning that they were less condensed and focused at the cellular surface and instead exhibited a more diffuse expression pattern [15].

7.3

Gliomas

7.3.1 Invasion and the Tumor Microenvironment The tumor microenvironment describes the environment around a particular tumor and consists of both cellular and non-cellular components [5, 52, 140]. It is important to note that the tumor cells and the other constituents of the microenvironment interact with one another, and this can influence the growth and spread of the tumor [52]. In gliomas their most conspicuous ability is their invasive properties within the central nervous system, which are typically very inhibitory to cellular movement. The high mortality rate of patients with high-­ grade gliomas is explained by the fact that gliomas uniquely invade the central nervous system [109, 126]. The neural ECM is usually thought of as an inhibitory environment, one that is not conducive to large-scale reorganization or remodeling; this has been attributed to the high presence of CSPGs [107, 113]. Gliomas are able to circumvent this inhibitory barrier. One of the main ways gliomas are able to do this is through the secretion of molecules that facilitate cell adhesion and movement, which include fibronectin and collagen [12, 22, 30, 44, 45, 99, 106]. Other ECM molecules have been demonstrated to regulate the phenotypic characteristics of gliomas including laminin, vitronectin, and tenascin­C. Using in vitro assays, it has been reported that glioma cells produce and secrete laminin [87, 103]. Expression of the glycoprotein, vitronectin, is correlated with the glioma grade, and its expression has been linked to increased cell survival of glioma cells [131]. Similarly, tenascin-C expression is also linked to glioma grade, and its expression is thought to be involved in mediating cell adhesion, migration, and cell dispersion [57,

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152]. Gliomas have also been shown to contain high levels of other ECM components like osteopontin, secreted protein acidic and rich in cysteine (SPARC), and thrombospondin [10]. Expressions of B/b, neurocan, and versican are also increased in glioma samples [100, 132, 134]. Enhanced expression of MMPs is characteristic of many tumor types, including gliomas. These enzymes degrade parts of the ECM, which then allows for the glioma cells to move through the ECM and infiltrate surrounding tissues. The MMPs that are upregulated in glioma cells include MMP-2, 3, 7, 9, 12, 13, 14, 16, 19, and 26 [33, 46, 63, 71, 73, 76, 85, 92, 106, 111, 115, 117, 118, 137, 138, 141, 148–150]. Other enzymes that are also responsible for the invasive properties of glioma cells are cathepsin B and urokinase-­ type plasminogen activator [11, 46, 71, 106, 115, 118, 141, 149] and heparanases and sulfatases [82]. In human gliomas, the overexpression of the forkhead box m1b (Foxm1b) factor leads to the enhanced invasion of glioma cells through an increase in transcription of the MMP-2 gene [32, 80]. Growth factors like epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), and transforming growth factor-β (TGF-β) have been reported to mediate glioma cell invasion [28]. Glioma cells commonly display mutations or amplifications in the EGF receptor (EGFR) gene, and there is an increased presence of this receptor on the surface of the tumorigenic cells [81, 96]. Interestingly, the activation of EGFR and extracellular signal-regulated kinase (ERK) is thought to result in an increase in the expression of fibronectin [38, 130, 155], which likely underlies the increase in migration exhibited by glioma cells. Hepatocyte growth factor (HGF) is commonly overexpressed in glioma cells, and, as a result, cell migration pathways are activated, which leads to the enhanced movement of these cells [47]. Similarly, insulin-like growth factor (IGF) is also overexpressed in these tumorigenic cells, and, in specific, an increase in expression of IGFBP2 leads to an upregulation of genes that are involved in cancer cell invasion, including MMP-2 [139]. High levels of the angiogenic fac-

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tor, angiopoietin-2 (Ang2), are detected in more invasive areas of gliomas, and this enhanced expression induces upregulation of MMP-2 both in vivo and in culture assays [50, 61, 69, 71]. The cell surface chemokine receptor, CXCR4, is also highly expressed in invasive glioma cells [36], and when the receptor interacts with a specific ligand, the Akt and ERK1/2 signaling pathways are activated, which affords glioma cells an increase in survival and cell division. This results in a more invasive phenotype [112, 142]. The canonical hyaluronan receptor, CD44, activates Rac1, which leads to a dramatic restructuring of the actin cytoskeleton within glioma cells. This receptor can be cleaved by ADAMTS10, and the product increases the invasive properties of glioma cells [3, 10, 91]. Rac not only facilitates the rearrangement of the actin cytoskeleton but is also known to increase cell motility [16, 110]. To demonstrate this, investigators inhibited Rac expression and found that glioma cell invasion was decreased [26, 29]. Rac does not work independently to mediate such important events; it has  been reported that Rac works with the polypeptide P311 [84, 88]. The nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is detected at high levels in glioma cells and has been hypothesized to afford these glioma cells an enhanced cell survival rate [114]. Glioma cells also exhibit changes in expression of cell adhesion molecules. For example, glioma cells express focal adhesion kinase (FAK) at higher levels than non-tumor cells [51, 56, 135], which has been linked to increases in cell proliferation [79, 151]. On the other hand, some cell adhesion molecules may exhibit decreased levels of expression in glioma cells. Expression of neural cell adhesion molecule (NCAM), for example, is reduced in glioma cells, which allows them to separate from neighboring cells and disperse into surrounding tissues [101, 116]. Cell surface integrin receptors that help join cells to one another are upregulated in glioma cells; specifically, this includes integrin α3β1, αvβ1, αvβ3, and αvβ5 [74]. Other cell adhesion molecules that display abnormal expression patterns in gli-

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oma cells include adhesion molecule on glia/β2 subunit of Na, K-ATPase (AMOG/β2), ephrin receptor tyrosine kinases (EphB2–B3), fibroblast growth factor inducible 14 receptor (Fn14), and protein tyrosine phosphatases zeta/beta [37, 94, 95, 121, 129]. Cadherin molecules also work by joining adjacent cells to one another to form structures like adherens junctions. Any changes to the structure and stability of these junctions result in an increase in the movement and invasiveness of glioma cells [4]. Glioma cells that express high levels of E-cadherin are phenotypically highly invasive [77], while cells that contain high levels of N-cadherin demonstrate the opposite, in that they are less invasive [19, 93].

7.3.2 Glioma-Initiating Cells (GICs) Gliomas are highly resistant to current therapeutic interventions, and, as a result, patient mortality rates are still quite high [126]. The invasive properties of gliomas are key in their therapeutic resistance; however, also key is the existence of glioma-initiating cells (GICs) within these tumors. In terms of the cellular composition, GICs that are present within the tumor are molecularly distinct from other cells found within the glioma. More specifically, the GICs are capable of self-renewing and exhibit multipotency, which means that they can differentiate into any subpopulation of cells found within the CNS as well as within the tumor itself. After orthotopic transplantation, these stem cells possess the ability to form a tumor that physically matches the parental tumor [25, 39, 123]. The GICs express many of the same proteins as those that are detected within normal stem cell niches [34] including laminin [122], tenascin-C [2, 40], members of the lectican family of CSPGs [68], and phosphacan [1, 2] as well as members of the integrin family [122]. Furthermore the GICs tend to be localized near vasculature within the tumors [43, 122]. It is important to note that the vasculature may develop due to the tumor presence itself, or the GICs might possess the ability to differentiate into endothelial cells, thus forming new blood vessels [108].

7.3.3 A  Key Role for B/b in the Glioma Microenvironment As noted above the interactions with tumors are complex within the tumor microenvironment; however, B/b presents as a uniquely intriguing target within this complex environment, and its specific roles are detailed below.

7.4

The Role of B/b in Gliomas

7.4.1 B/b Expression in Gliomas Enhanced levels of B/b expression have been detected in human glioma samples, including oligodendrogliomas, all examined grades of astrocytomas, and gliosarcomas, relative to normal brain tissue and tissues derived from non-glioma tumors [42, 67]. More specifically, this increase in B/b expression was detected within the ECM as well as the cytoplasm of glioma cells. Importantly, within higher grades of astrocytomas (grades III and IV), B/b staining was more dispersed, indicative of an increase in infiltration, compared to lower grades [83]. Glioma cell lines (e.g., 9L, CNS-1, and C6) that are propagated under normal culture conditions do not express B/b, but if they are grown as intracranial grafts, then they express B/b. This phenomenon was not noted in cells taken from noninvasive tumors [67]. In rodent and human glioma samples, B/b is cleaved, and the resulting products are a N-terminal fragment that includes the hyaluronan-­ binding portion of the protein core (~50–60 kDa) and a C-terminal fragment (~90–100 kDa). Full-­ length B/b runs at ~160 kDa [133, 154]. Gary and colleagues set forth the hypothesis that B/b modulates the invasiveness of gliomas [41]. To examine the properties and behaviors of gliomas further, investigators introduced B/b into CNS-1 cells in vitro through transfection. These cells were transfected either with a green fluorescent protein (GFP) control, full-length B/b, the C-terminus cleavage fragment of B/b, or the N-terminus cleavage fragment of B/b. These cells

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were then injected into rats to assess the resulting tumors. The rats that were injected with the CNS-1 cells transfected with the various forms of B/b, exhibited  a lower survival rate than those rats that received the control cells. Furthermore, the B/b-derived tumors were more invasive and were highly vascular compared to the control tumors. This led to the conclusion that B/b enhances the aggressive properties of gliomas [98]. While it was established that full-length B/b is overexpressed in human glioma samples [42, 67], it was then identified that two novel isoforms of B/b were present in tumor tissues [133]. The two new isoforms were denoted as B/b∆g with a molecular mass of 150 kDa that was found within the membrane fraction and B/bsia, which had a molecular weight higher than 150 kDa and was located in both the membrane and soluble fractions. It is important to note that the benign tumors that were assayed did not express these B/b isoforms, thereby indicating that perhaps these identified forms of B/b could be used as indicators of tumor grade. Furthermore, these isoforms were specifically identified in gliomas and were not present in tissues derived from patients with epilepsy or Alzheimer’s disease. Neither isoform of B/b was found within individuals over 1 year of age. Only the B/b∆g form was faintly detected in samples harvested from embryos at 16 weeks of gestation to infants aged 19 days [133]. Through biochemical analyses, it was determined that these new isoforms of B/b were not cleavage fragments, and the peptide sequences of these forms were identical to the full-length protein, thereby indicating that these forms were derived from the same mRNA transcript. Attention was then focused on determining how these isoforms were molecularly distinct from the full-length protein. Deglycosylating enzymes were used to remove N-linked and O-linked sugars from the protein core as well as CS chains (through the use of chondroitinase), and this revealed that the B/b∆g form was underglycosylated. Further work determined that this particular isoform associates with the cell membrane in a manner distinct from other B/b forms.

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Importantly, this does not require calcium, interaction with the CS chains, nor the N-terminal domain of B/b, affirming that the molecular association with the membrane is unique for this specific isoform. The B/bsia form is an over-sialylated version of the protein and is generated when there is an increase in the amount of sialic acid added to O-linked carbohydrates [133]. Having established that B/b is expressed at high levels in gliomas, the next step was to ascertain which specific cellular population contained the highest amounts of B/b. Human glioblastoma tumor sections were analyzed, and B/b was found around cells that expressed Olig2 and CD133 markers, both of which are indicative of highly tumorigenic cells [20, 49]. Interestingly, these markers are also detected within GICs. To examine B/b expression in these cells, researchers utilized two GIC lines: 0627 and 0913. They determined that both B/b protein and mRNA were present in these cell lines, although the 80–90  kDa C-terminal cleavage fragment was only expressed in the 0627 cells. Interestingly, B/b knockdown did not alter any of the assayed physical properties of the glioma-initiating cells including proliferation rate, viability, adhesive properties, migration, and invasion. Based on these results, it does not appear that B/b is needed for the GICs to behave normally nor for the maintenance of their characteristic physical properties, so likely B/b works in this cell population during the later stages of glioma pathogenesis [35].

7.4.2 C  leavage of B/b in Gliomas Leads to Increased Invasiveness To address the ability of B/b to promote invasiveness, cultured 9L cells were transfected with either the full-length protein or the N-terminal fragment described above. It is important to note that the 9L cell line is characterized as a noninvasive cell line. 9L cells that expressed either the full-length form of B/b or the N-terminal fragment displayed a higher degree of motility and invasion as compared to cells that were trans-

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fected with GFP. Of particular importance, when these cells were injected into rats, only the tumors that expressed the N-terminal cleavage fragment were able to invade surrounding brain tissue. This was not noted in tumors that expressed the full-length form of B/b, thereby suggesting that the cleavage of B/b is a key event that mediates glioma cell invasiveness in rat models [154]. This finding prompted the investigation into which molecule cleaves the lectican family member. This was addressed through the generation of an antibody against the putative cleavage site at Glu395-Ser396 within B/b [86]. The cleavage site is homologous to the well-characterized site in another CSPG, aggrecan [156]. Cleavage of aggrecan at that particular site is regulated at least partially by ADAMTS4 [128]. The resulting antibody exclusively recognizes the N-terminal fragment of B/b and is referred to as B50. Through use of the invasive CNS-1 cell line, cleavage activity was detected in culture, and most of the resulting product was detected in the media and, thus, was soluble. Investigators then aimed to determine the proper conditions for B/b cleavage by altering calcium, zinc, and sodium chloride levels in addition to pH and temperature. Administration of calcium chelators, and metalloproteinase inhibitors to the cultures, diminished the cleavage of B/b. From this work, Matthews and colleagues examined the potential role of ADAMTS4 in mediating the cleavage of B/b. They concluded that ADAMTS4 not only was expressed in CNS-1 cells but was also capable of cleaving B/b. This work pinpoints a critical role for ADAMTS4 to regulate B/b cleavage and, by extension, the invasive behavior displayed by glioma cells [86]. To directly assess if this cleavage event is necessary for the pro-invasive properties seen in glioma cells, a mutant construct in which B/b was not cleaved was introduced into CNS-1 cells. Tumor spheroids were created and then applied to organotypic slice cultures, and migration of the cells was examined. The spheroids containing the wild-type form of B/b migrated across the slice cultures more than those that expressed the mutant form of B/b that could not be cleaved. The CNS-1 cells that were transfected with wild-type

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B/b were implanted into rats intracranially, and the resulting tumors were more invasive, dispersed, and larger compared to those tumors that formed when the CNS-1 cells transfected with the mutant form of B/b were injected. Rats that had tumors that were more invasive exhibited a decreased survival rate compared to their counterparts [134].

7.4.3 M  olecular Mechanisms: How B/b Cleavage Promotes Invasiveness Having determined that the cleavage of B/b promotes glioma cell invasion, the mechanisms underlying this were next explored. B/b was introduced into glioma cells (U87MG, U373MG, and CNS-1 cells) through transduction in culture. Expression of B/b enhanced glioma cell adhesion to specific substrates: fibronectin, collagen, and hyaluronic acid, but this was not noted when laminin and poly-L-lysine were used. Moreover, investigators probed glioma cell motility and reported that glioma cells expressing B/b were more mobile in response to hyaluronic acid and fibronectin substrates, in comparison with control cells that did not express B/b. B/b cleavage was required for the adhesion between B/b and fibronectin and hyaluronic acid. To provide further support, the glioma cells were added to organotypic slice cultures to measure the amount of cell dispersion. Glioma cells that expressed either the full-length form of B/b or the N-terminal cleavage fragment of B/b exhibited a significant increase in cell movement compared to those cells that expressed the form of B/b that was resistant to cleavage [62]. The expression of a number of cell adhesion molecules is altered in glioma samples [4, 19, 37, 51, 56, 74, 77, 79, 93, 94, 95, 101, 121, 129, 135, 151, 157], and Hu and colleagues then focused on identifying which cell adhesion molecules might be involved in modulating glioma cell invasion. Glioma cells that expressed B/b and were plated on fibronectin displayed an increase in protein expression of β-3 integrin, a phosphorylated form of the β-3 integrin, and NCAM, in comparison

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with control cells that did not express B/b [62]. These results are in accordance with reports that β-3 integrin expression induces cell dispersion and spreading [143, 144]. It is known that both B/b and fibronectin are upregulated in gliomas [99, 133] compared to normal brain tissue [97, 102] and tumors that spread to the brain, but did not originate in the brain [67]. Specifically, fibronectin was found at the cell surface on glioma cells that possessed either the full-length form of B/b or the N-terminal cleavage fragment of B/b. The expressed fibronectin was organized in microfibrillar structures, which is thought to facilitate rearrangement of the ECM and promote movement of tumor cells [60, 72]. When U87MG glioma cells were incubated in conditioned media that contained either secreted full-length B/b or the N-terminal cleavage fragment of B/b, there was an increase in the amount of phosphorylated EGFR and phosphorylated ERK1/2 compared to control cells. If the glioma cells were treated with an EGFR inhibitor, then phosphorylation was inhibited, and, correspondingly, fibronectin mRNA levels decreased. Importantly, as a result of the treatment with this inhibitor, the B/b-expressing glioma cells did not adhere as well to fibronectin relative to cells that were treated with a control empty vector [62]. This work is consistent with reports that the expression of EGFR is increased in glioma cells [81, 96]. Additionally, the results presented by Hu et  al. [62] corroborate previous demonstrations that the activation of EGFR and ERK induces an increase in fibronectin expression [38, 130, 155]. Precisely how fibronectin and B/b might associate with one another was directly addressed through co-immunoprecipitation and dot blot assays, in which it was shown that fibronectin binds to the N-terminal cleavage fragment of B/b, but not the full-length form of the protein. This clearly shows that the cleavage of B/b is an important event that is required for binding to fibronectin, which results in the enhancement of glioma cell movement [62]. The work presented by Hu and colleagues was supported by another set of experiments where U251 and U87 glioma cells were induced to express B/b, which resulted in an increase in the

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adhesion of glioma cells to fibronectin and an overall increase in motility of the glioma cells [83].

7.4.4 I mpact of B/b Knockdown on Glioma Cells To further pinpoint the critical role that fibronectin plays in mediating glioma cell motility, siRNA constructs were made to knockdown fibronectin expression. As a result, glioma cells adhered less to hyaluronan and fibronectin and additionally were less motile when plated on these substrates. Phenotypically, these glioma cells now presented the same as the control cells in terms of adhesion and motility [62]. To more thoroughly analyze how B/b is involved in regulating glioma cell behavior, U251 cells were first transduced to express B/b, and then the protein was knocked down using shDNA. Due to the knockdown of B/b, these glioma cells displayed a decrease in the rate of division and reductions in the following properties: invasiveness, migration, and dispersion or spreading distance, in direct comparison to the shDNA and mock controls. To examine resulting tumor growth in vivo, the transduced cells were introduced into nude mice. The mice that received the B/b knockdown cells developed tumors that were less infiltrative and less dispersed relative to the mice that received the control cells. This work further defined the role of B/b role in regulating glioma cell migration and invasion [83]. Dwyer and colleagues then aimed to elucidate what occurs when B/b is knocked down in intracranial gliomas. To this end, investigators transfected CNS-1 cells with B/b expression constructs at the same time as B/b knockdown constructs. After determining the knockdown efficiency, the generated CNS-1 cells were injected in the thalamus of rats. The tumors that formed after CNS-1 cells exposed to the shRNA construct specific to B/b displayed a reduction in overall volume and were less invasive compared to the tumors that developed when a control shRNA construct was introduced into the CNS-1 cells. In light of this, the survival rates of the rats injected with the B/b

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knockdown cells were higher than those rats that received the control cells [35]. This body of evidence suggests that B/b expression in gliomas results in increased motility and invasion [35, 83]. As stated above, B/b expression was detected in GICs [20, 35, 49], but the question as to how B/b functions in this cell population was next addressed. shRNA constructs were introduced into GICs and then were injected into the striatum of nude mice to examine the properties of the tumors that were generated as a result. The GICs that expressed the knockdown constructs to reduce B/b expression formed a tumor that was smaller in volume and was less invasive relative to the cells that contained control constructs [35]. Therefore, it does appear that B/b mechanistically functions in the same capacity in the glioma-­initiating cells as in glioma cells to promote invasion, spread, and migration.

7.5

Future Directions

B/b is a key molecule present within the tumor microenvironment of gliomas that works to promote cell invasion and movement [35, 41, 62, 83, 86, 98, 133, 134, 154]. Due to the fact that glioma cells possess the ability to infiltrate the normally inhibitory ECM, patient prognosis and response to current treatment options are quite poor [126]. In addition, work suggests that B/b contributes to tumor vascularization, but the mechanism by which it does this is completely unknown [98]. Future work investigating the interaction between B/b and other cells in the tumor microenvironment such as pericytes and vascular endothelial cells is clearly necessary. In addition, future studies need to be aimed at creating treatments that specifically target the GIC population. These cells are capable of self-­renewal and also are able to form all of the cells within a glioma [39, 123]. Importantly, these cells create and maintain an environment that fuels tumor development, which not only is likely responsible for driving the initial establishment of the tumor but also explains why relapses might occur [6, 27, 78]. Therefore, treatments tailored to targeting the GICs might provide promising new avenues that could lead to better patient survival rates. B/b is

an intriguing target in this regard as it seems to be an important component of the stem cell niche. A complicating factor in treating individuals with gliomas is the fact that there is a great degree of molecular heterogeneity in these types of tumors. More specifically, cell adhesion molecules, ECM constituents, enzymes, and growth factors are just some examples of molecules that may underlie glioma pathogenesis. Importantly, these molecules work together to create an intricate and complex tumor microenvironment. In light of this, the best way to devise treatments is to precisely pinpoint how these molecules work together to contribute to the development and maintenance of gliomas in addition to defining the specific role of each of these molecules. This will give us a more complete picture as to how these tumors develop, thereby, providing us with the information necessary to generate more effective treatments.

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8

Thrombospondin in Tumor Microenvironment Divya Ramchandani and Vivek Mittal

Abstract

Thrombospondins (TSPs) are multifaceted proteins that contribute to physiologic as well as pathologic conditions. Due to their multiple receptor-binding domains, TSPs display both oncogenic and tumor-suppressive qualities and are thus essential components of the extracellular matrix. Known for their antiangiogenic capacity, TSPs are an important component of the tumor microenvironment. The N- and C-terminal domains of TSP are, respectively, involved in cell adhesion and spreading, an important feature of wound healing as well as cancer cell migration. Previously known for the activation of TGF-β to promote tumor growth and inflammation, TSP-1 has recently been found to be transcriptionally induced by TGF-β, implying the presence of a possible feedback loop. TSP-1 is an endogenous inhibitor of T cells and also mediates its immunosuppressive effects via induction of Tregs. Given the diverse roles of TSPs in the tumor microenvironment, many therapeutic strategies have utilized TSP-mimetic D. Ramchandani · V. Mittal (*) Department of Cardiothoracic Surgery, Weill Cornell Medicine, New York, NY, USA e-mail: [email protected]; [email protected]

peptides or antibody blockade as anti-­ metastatic approaches. This chapter discusses the diverse structural domains, functional implications, and anti-metastatic therapies in the context of the role of TSP in the tumor microenvironment. Keywords

Thrombospondin · Cancer · Microenvironment · CD36 · CD47 · Angiogenesis · TGF-β · Adhesion · Spreading · Invasion · Migration · Dormancy · Pre-­ metastatic niche · Metastasis · Immune system

8.1

Introduction

Thrombospondins (TSPs) are extracellular, calcium-­binding, oligomeric, adhesive glycoproteins that mediate cell-cell and cell-matrix interactions [1]. All TSPs (TSPs 1–5) are expressed at varying levels in many cell types, including endothelial cells, fibroblasts, smooth muscle cells, adipocytes, and macrophages, and exist as a component of the extracellular matrix [2, 3]. TSP-1 promoter has binding sites for transcription factors (OCT-1, IRF-1, AP2, Egr-1, STAT1, PPAR, ATF-1, etc.), indicative of a complex reg-

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 A. Birbrair (ed.), Tumor Microenvironment, Advances in Experimental Medicine and Biology 1272, https://doi.org/10.1007/978-3-030-48457-6_8

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ulatory mechanism [2]. TSP upregulation has been observed in physiological processes, including wound healing and repair [4–6], and has emerged as a major player in carcinogenesis [7, 8]. TSPs exhibit context-dependent functional roles; for example, TSP-1 has an inhibitory effect on melanoma growth [9], whereas TSP3 is associated with worse prognosis in osteosarcoma [10]. TSP4 has been associated with tumor suppression in colorectal cancer [11] and increased invasion in breast cancer [12]. TSP2 is associated with increased metastasis in prostate cancer [13] and lung cancer [14] yet exhibits an inhibitory role in angiogenesis and tumor growth in squamous cell carcinoma [15]. Here, we summarize the structural aspects of TSPs, with specific emphasis on the role of TSP-1  in the tumor microenvironment. For general descriptions of TSP in cancer progression, readers are referred to several excellent reviews [16–18]. Thrombospondin, specifically TSP-1, has a multifaceted role in the tumor microenvironment which regulates cancer progression. In addition to its effects on tumor cells, TSP-1 affects tumor stromal cells including endothelial cells, fibroblasts, macrophages, dendritic cells, and T cells. TSP-1 mediates these effects via the interaction with its receptors, CD36 and CD47, or through direct regulation of TGF-β and impacts key signaling pathways. Various functional roles of TSP-1 in the tumor microenvironment are shown in Table 8.1.

8.2

Thrombospondin Structure

TSP-1 is a multifunctional protein, and its diverse biological activities have been mapped to specific domains that interact with different cell surface receptors. TSPs have a conserved carboxyl-­ terminal “signature domain” which consists of an EGF-like domain, 13 calciumbinding type 3 repeats, and a region homologous to L-type lectin domain. The amino terminus is variable by virtue of possessing a laminin G-like amino-terminal domain and a helical coiled-coil domain that is responsible for the oligomeriza-

tion. The oligomerization of TSP subunits is mediated by disulfide bonds between adjacent cysteine residues [46]. Based on their oligomeric assembly, TSPs are classified into subgroup A. Trimers, TSP-1 and TSP-2, and subgroup B. Pentamers, TSP-3, TSP-4, and TSP-5 (or COMP). An array of multiple repeats including TSP type 1 repeats (TSR1), TSP type 2 repeats (TSR2, EGF-type repeats), and TSP type 3 repeats (TSR3, seven continuous calcium-binding repeats) exist between the globular carboxyl and amino-terminal domains in TSPs (Fig. 8.1) [47]. TSPs in subgroup B lack type 1 TSRs present in TSP-1 and TSP-2. Since TSRs are linked to antiangiogenic activities [48–50], TSPs in subgroup B are believed to lack antiangiogenic properties.

8.3

Thrombospondin Receptors

The numerous domains in TSP (Fig. 8.1) mediate a wide array of functions by virtue of binding to different classes of cell surface receptors described below.

8.3.1 Low-Density Lipoprotein Receptor-Related Proteins, Sulfatides, and Proteoglycans The amino-terminal domain (residue 1-214) of TSP binds to low-density lipoprotein receptor-­ related protein (LRP) to mediate its internalization and degradation, and this receptor-mediated endocytosis requires heparan sulfate proteoglycans [51, 52]. The N-terminal also binds to a group of anionic molecules, the sulfatides [53]. The interactions involving sulfated glycoproteins and heparin-binding domain in TSP are critical in melanoma cell spreading [54]. Melanoma cell lines that produce glucuronosyl 3-sulfate-­ containing glycolipids and glycoproteins are selectively capable of spreading via binding to TSP, whereas C32 melanoma cells which do not produce these sulfated glucuronosyl oligosaccharides lack spreading [55].

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Table 8.1  Functional roles of TSP-1 in tumor microenvironment Cell-type TSP-1 function affected Adhesion Cancer cells

Invasion and migration Angiogenesis

Immune system

Cancer cells Endothelial cells Endothelial cells Cancer cells

T cells Tregs Macrophages NK cells Dendritic cells

Effect Increased adhesion Decreased adhesion

Increased motogenic effects Apoptosis Inhibition of neovascularization Inhibits proliferation and migration Inverse correlation with VEGF expression Inhibits T-cell activation T-cell migration Induction of CD4+ Tregs Activation and recruitment Proliferation Affects maturation, trafficking, and anti-tumor responses

Mechanism Integrin, sulfated glycoconjugates or CD36-mediated TSP-1 adhesion Upregulation of urokinase plasminogen activator receptor Increased MMPs expression via integrin signaling and via TGF- β

Reference [19–24]

[25–27]

Via dephosphorylation and CD36 via FGF-2 via bFGF no known direct link

[28–33]

CD47 and integrin-associated protein heparan sulfate proteoglycans-mediated inhibition of TCR signaling. TSP-1 also inhibits H2S-mediated MAPK signaling SNAIL-induced EMT leading to metastasis TLR-4 pathway, plasminogen activator inhibitor-1 Via TGF- β Via CD47

[34–45]

Fig. 8.1  Domain architecture of TSP-1 and TSP-2. Different domains and active peptide sequences that bind to different receptors are depicted

8.3.2 CD36 or GPIIIb CD36 (cluster of differentiation 36) is a membrane glycoprotein expressed on the surface of a wide range of cells including platelets, monocytes, megakaryocytes, endothelial cells, mam-

mary epithelial cells, and cancer cells. The binding of TSP to CD36 is facilitated through VTCG sequence in the TSRs of TSP. Interaction of TSP and CD36 has been implicated in various functional outcomes including angiogenesis inhibition [56, 57], activation of TGF-β1 [58], plate-

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let aggregation [59], platelet-tumor cell adhesion [60], etc. Apoptosis of endothelial cells via TSP-1 is mediated through CD36 receptor by the activation of CD36-p59fyn-caspase 3-p38MAPK cascade, c-Jun N-terminal kinases, and Fas/Fas ligand [56, 57, 61].

8.4

Functional Roles of Thrombospondin

8.4.1 Cell Adhesion and Motility

Interaction of TSP with various cell surface receptors, including integrins and sulfatides, is involved in mediating cellular attachment, 8.3.3 Integrins spreading, and cell motility. The peptide sequence WSPW from the second type 1 repeat The integrin-binding RGD domain in TSP is in TSP and the heparin-binding domain in the located in the seventh type 3 calcium-binding N-terminus promote endothelial cell adhesion repeat. Apart from binding to αvβ3 and αIIbβ3 on while inhibiting basic fibroblast growth factorthe platelet surface, TSP also binds to αvβ3, mediated endothelial cell chemotaxis [74]. αIIbβ3, α3β1, α4β1, and α5β1 [62]. Integrin-­ TSP-1 is known to mediate cell attachment in mediated TSP functions include insulin-like breast cancer, melanoma, small cell lung cancer, growth factor signaling [63], neurite growth [64], etc. via its interaction with its receptors includphagocytosis [65], and cancer cell spreading via ing integrins and sulfatides [19, 20, 22, 75]. α3β1 [19]. Contrary to this, TSP-1 type 1 repeats are also known to inhibit endothelial cell migration through β1 integrins via a PI3k-­dependent signaling [76]. Thus, TSP-1 may act differently 8.3.4 Integrin-Associated Proteins (IAP or CD47) and Other depending on the receptor binding and downC-Terminal Receptors stream signaling. The two active peptides in the C-terminal of TSP that support cell attachment are RFYVVMWK and IRVVM [66, 67], which bind to the 52 kDa receptor CD47 [68]. IAP or CD47 has been associated with mediating cell adhesion properties of TSP.  TSP-CD47 interaction affects integrin-­ mediated activities through modulation of αvβ3 on melanoma cells, αIIbβ3 on platelets, and α2β1 on vascular smooth muscle cells [69–71]. TSP-CD47 interaction reduces inflammation [72]. TSP bound to CD47 on T-cell surface induces the expression of BNIP3 (Bcl-2/adenovirus E1B 19 kDa-interacting protein) to mediate T-cell apoptosis and thus reduce inflammation [72]. Apart from calcium-dependent integrins and CD47, TSP also binds to another receptor termed Mr 80,000/105,000 at its C-terminus that has two component molecular weights: 80  kDa and 105 kDa [73]. This receptor binds to TSP in a Ca2+- and Mg2+-dependent fashion and is cross-­ reactive with members of the β1or β3 integrin receptor families [62, 73].

8.4.2 Angiogenesis TSP serves as a naturally occurring inhibitor of angiogenesis, which is critical for both tumor growth and metastasis. TSP negatively regulates angiogenesis either by inhibiting proliferation of endothelial cells [77] or by inducing their apoptosis [57]. Binding of TSP-1 to CD36 promotes downstream activation of pp59fyn, p38MAPK, and caspase-3 to induce apoptosis in endothelial cells [57]. TSP-1 also inhibits lymphangiogenesis by binding to CD36 on monocytes and inhibiting lymphangiogenic factors: VEGF-C and VEGF-D [78]. TSP-1 inhibits the expression of platelet-endothelial cell adhesion molecule-1 in brain endothelial cells, causing altered cell-cell interactions and reduced formation of vascular networks [79]. However, a recent study demonstrated increased TSP-1 expression in the vasculature of angiogenic tumors [80].

8  Thrombospondin in Tumor Microenvironment

8.4.3 Blood Flow TSP-1 produced by vascular cells is present in the plasma and regulates blood flow to tissues. TSP-1 prevents NO/cGMP signaling to relax vascular smooth muscles, thereby regulating blood vessel tone and blood pressure to tissues [81]. Moreover, TSP-1 also blocks anti-thrombotic activity of NO/cGMP signaling axis [82]. These inhibitory responses require both TSP-1 receptors: CD36 and CD47 [83, 84]. Deficiency of either CD47 or TSP-1 alters the resting blood pressures and vasoactive stress responses [85]. Consistent with this notion, administration of a recombinant antiangiogenic domain of TSP-1 (three TSP-1 type 1 repeats or 3TSR) reduced tumor blood flow in an orthotopic model of pancreatic cancer [86]. TSP-1 overexpression by tumor has been found to moderately reduce tumor blood flow in response to vasoactive agents like NO or epinephrine in a melanoma model [87]. TSP-1 also alters blood flow to ischemic tissues via ROS production [88]. TSP-1-CD47 binding attenuates vasodilation and blood flow to injured tissues via activation of PKC and p47phox that promotes Nox1 to produce superoxide anions, which regulate vascular tone [88].

8.4.4 Activation of TGF-β TGF-β is a multifaceted cytokine involved in a multitude of cellular functions including cell growth, differentiation, immune modulation, and inflammation. TGF-β activation requires TSP-1 central type 1 repeats containing amino acids KRFK. TSP-1 interacts with the LSKL sequence of the N-terminal domain of latency-associated peptide (LAP) of latent TGF-β, and instead of cleaving LAP for the activation (as seen with plasmin-mediated TGF-β activation) [89], TSP-1 induces a conformational change at this site to improve the accessibility of TGF-β to its receptor [90]. LAP with deleted LSKL is unable to bind or activate TSP-1 and thus is unable to provide latency to TGF-β [91]. The critical role of TSP-1  in promoting TGF-β activation has been demonstrated in pathologies including tumor

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growth and inflammation. TGF-β itself has been recently shown to increase TSP-1 expression in glioblastoma via SMAD3-dependent direct transcriptional activation [92]. Increased TSP-1 expression then leads to higher invasion through its interaction with CD47 receptor [92].

8.5

Thrombospondins in Carcinogenesis

Thrombospondins are both expressed by malignant cells and non-malignant cells in the tumor microenvironment [2]. In addition to angiogenesis modulation, the effects of TSP-1 on tumor progression are multifaceted and sometimes opposed, depending on the molecular and cellular interactions of its domains and cell surface molecules [17]. For instance, in melanoma, TSP-1 exhibits inhibitory effects of tumor cell migration (chemotaxis and haptotaxis) and proliferation. Inhibition of tumor cell proliferation was observed in both CD36-negative (more sensitive to TSP-1 effects) and CD36+ melanoma cells (less sensitive to TSP-1 inhibition), which is in contrast to the effects of TSP-1 on endothelial cells [93]. This anti-proliferative effect was ameliorated by inhibition of tyrosine kinase (phenotype reversed by herbimycin) or phosphatase (phenotype reversed by vanadate) [93]. One such tyrosine phosphatase receptor for TSP-1 was later found to be CD148 receptor-type protein tyrosine phosphatase, which negatively regulates cell growth upon TSP-1 binding [94]. TSP-1 deficiency reduces survival in the absence of p53 [95]. High and prolonged exposure of tumor cells to stromal TSP-1 has been found to lead to an increased breast cancer progression [96]. TSP-1 is also known to enhance invasion of oral squamous cell carcinoma cells by the upregulation of MMP9 through its interaction with integrin receptors [25]. As a tumor suppressor, TSP-1 null mice have been found to have larger mammary tumors, with higher METs and higher TGF-β expressions compared to wild-type mice [97]. TSP-1 induces monocyte-mediated killing of squamous carcinoma [98] and macrophage-­ mediated cytotoxicity of tumor cells [39, 40].

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TSP2 is a more potent inhibitor of angiogenesis than TSP-1 and reduces tumor growth, even in the presence of high VEGF levels [15]. Thus, thrombospondins can act as both suppressors and promoters of carcinogenesis in the tumor microenvironment.

D. Ramchandani and V. Mittal

of TSP-1 allowed formation of distended capillaries and sinusoidal vasculature, enhancing tumor growth and expression of VEGF, VEGFR2, and MMP-9. In contrast, TSP-1 overexpression delayed tumor growth associated with reduced vascularization [102]. TSP-1 also mediates antiangiogenic activity via endothelial cell apoptosis. Tumor cell-derived TSP-1 is critical for inducing 8.6 Thrombospondins in Tumor cyclophosphamide-dependent endothelial cell apoptosis. Consistent with this finding, a lack of Microenvironment TSP-1 abrogates sensitivity of tumors to low-­ The finding that TSP-1-deficient mice exhibit dose cyclophosphamide [103]. TSP-1 has oppoincreased growth of implanted tumors provided site effects on endothelial cells and perivascular direct evidence on the functional contribution of supporting cells (vascular smooth muscle cells host TSP in carcinogenesis. In either homozy- and pericytes) [104]. TSP-1 along with platelet-­ gous or heterozygous p53 null mice, which derived growth factor (PDGF) and fibronectin develop spontaneous carcinomas, TSP-1 deletion induces chemotaxis of vascular smooth muscle reduced overall survival and increased progres- cells, useful in response to arterial injury [105]. sion of the disease and the associated mortality TSP-1 is very important in the proliferation and [95]. With numerous receptor-mediated interac- migration of pericytes during retinal vascular tions and varying expressions of TSPs in cancer, development [106]. This effect of TSP-1 on periactivities of TSPs in regulating different func- cytes may have a paradoxical consequence as tions in the tumor microenvironment are detailed type 2 pericytes are involved in angiogenesis of ischemic tissues and may also improve blood below. flow in cancer [107]. The antiangiogenic activity of microenviron8.6.1 Angiogenesis and Tumor mental TSP-1 is necessary to inhibit tumor outGrowth growth. However, tumors generally overcome this inhibition to sustain their survival and mainTSP-1 and TSP-2 are both endogenous inhibitors tain malignant progression. For example, Ras-­ of angiogenesis. Overexpression of TSP-1 inhib- mutant tumors inhibit fibroblast TSP-1 production its malignant tumor progression and metastasis via GPCR/SIP and Id1, in order to overcome the associated with angiogenesis suppression [99]. suppressive effects of TSP-1 [108]. Loss of p53 Similarly, TSP-2 overexpression by fibroblasts alleles leads to reduced TSP-1 secretion and can also inhibit growth of squamous cell carcino- increased VEGF secretion that facilitates maligmas, malignant melanomas, and Lewis lung car- nant transformation [109]. Consistent with this cinomas by angiogenesis inhibition [100]. TSP-2 observation, topical delivery of p53 to the lungs mediates endothelial cell apoptosis and inhibits of mice reduced tumor burden in the lungs in a the migration of endothelial cells to suppress B16-F10 melanoma model [110]. Tumor supneovascularization and, thus, tumor growth pression was associated with reduction in angio[101]. The antiangiogenic activity of TSP-1 has genesis and VEGF production, while TSP-1 been mapped to two domains in its sequence: levels and mice survival increased [110]. type 1 repeats (TSRs) and procollagen homology Additionally, in the absence of active MYC, region [28]. This antiangiogenic activity of TSP-1 introduction of TSP-1  in p53 null tumors supis not dependent on the presence of TGF-β-­ pressed angiogenesis in order to regress tumor activating sequence (RFK), whereas the effect on growth [111]. tumor cell growth and apoptosis depends on Stroma-derived TSP-1 has the potential to TGF-β [9]. In a breast cancer model, deficiency inhibit angiogenesis by suppressing NO signal-

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ing via the inhibition of guanylate cyclase and cGMP-dependent protein kinase [81]. TSP-1 mediates its effects via both CD47 and CD36 receptors, where CD47 is more sensitive and also critical for the effects through CD36 [112]. These effects of TSP-1 may lead to a reduction of tissue perfusion of other organs and increased perfusion of the tumor as the vasodilation in tumors is less prone to the acute effects of NO signaling, thereby mediating TSP-1 side effects in tumor progression [112]. Prosaposin, a potent inhibitor of tumor metastasis, stimulates TSP-1 in monocytes, which via binding to CD36 receptors on the tumor cells mediates tumor suppression [113].

tor inhibitor-1, and a reduction in plasmin activity, leading to increased cell attachment, enhanced flattened cell morphology, and cell spreading, thus increasing cancer metastasis [24, 124–126]. Peptides from the type I repeats of TSP, which consists of the sequence Trp-SerXaa-Trp, inhibit the binding of heparin and sulfatide to the amino terminus of TSP and promote cell adhesion of melanoma cells [127]. Different domains in TSP mediate tumor cell chemotaxis and haptotaxis. The carboxyl domain is responsible for TSP-induced haptotaxis, whereas NH2 heparin-binding domain of TSP affects only chemotaxis and not haptotaxis in cells [128].

8.6.2 C  ell Morphology, Adhesion, and Migration

8.6.3 Inflammation

TSP is an adhesive glycoprotein with multiple domains including N-terminal heparin-binding domain, COOH-terminal cell or platelet-binding domain, and RGDA sequence that mediate cellular adhesion, migration, and spreading [53, 54, 114–117]. CSVTCG in the type I repeats of TSP-1 promotes cell attachment [118]. A correlation between CSVCTG-specific TSP-1 receptor on gastric cancer cells and upregulation of MMP-9 has been associated with an aggressive tumor phenotype [119]. Expression of TSP-1  in mesangial cells has been found to be upregulated by signaling pathways involved in cell morphology and spreading. Src family kinases, ERK1/2 and small GTPase Rac-1, along with soluble factors in the serum upregulate the expression of TSP-1  in mesangial cells upon changes in cellular morphology [120]. TSP-1 is known to increase cancer cell invasion [121, 122]. Exogenous TSP-1 increases MMP-9 levels in endothelial cells, promoting their invasion and morphogenesis into tube-like structures [123]. Thrombospondin, via TGF-β, has been suggested to increase the expression of extracellular matrix-­ degrading proteases, urokinase plasminogen activator and plasminogen activa-

TSP-1 expression is elevated during inflammatory responses. TSP-1 promotes the production of inflammatory cytokines IL-6, IL-1β, and TNF-α via NF-kB pathway in human monocytes, and inhibition of TSP-1 expression reduces cytokine production [129]. CD47 induces both T- and B-cell death to reduce inflammation [130, 131]. This clearance of lymphocytes is mediated by the binding of both TSP-1 and TSP-2 (and not SIRP ligands) to CD47 through their COOH-terminal domain. Proapoptotic signals in T cells mediate the translocation of CD47-bound BNIP3 to the mitochondria to trigger a mitochondrial cell death [132]. Deficiency of either TSP-1, TSP-2, or CD47 limits the clearance of activated T cells [72]. CD47-­ TSP-­ 1 interaction is also responsible for the generation of CD4+ CD25+ Tregs from CD4+ CD25- naïve or memory T cells, which suppresses proliferation and cytokine production by Th0, Th1, and Th2 cells by a contact-dependent and TGF-β-independent mechanism to reduce inflammation [133]. A TGF-β-dependent mechanism of inflammation suppression by TSP-1 is also described. TSP-1-primed monocytes increase the expression of TGF-β, which induces Tregs to suppress inflammation [134]. In cancer, the role of TSP-1 is context dependent. For

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example, in inflammation-induced cancers like colorectal cancer, TSP-1−/− mice show a lower tumor burden, despite an increase in new blood vessels and increased tumor cell proliferation, compared to the wild-type mice [135].

8.6.4 TSP in Metastasis Microenvironment

D. Ramchandani and V. Mittal

Following dissemination of primary tumor cells to distant organs, DTCs are not immediately competent to initiate growth and can persist in a dormant state [143]. Tumor recurrence occurs when dormant cells in metastatic niches escape their quiescent state. Thus, it is important to understand the mechanisms that keep tumor cell outgrowth at bay or even help in killing these dormant disseminated tumor cells in the microvasculature of different organs like the bone marrow, lungs, or brain. Endothelial cell-derived TSP-1 in the metastatic niche has been shown to induce DTC dormancy [144].

Malignancies including breast, skin, and colon cancer secrete factors and extracellular vesicles, which systematically reprogram distant organs including the lung to generate “pre-metastatic niches” (PN) [136]. Studies in mouse models have shown that these PNs characterized by host 8.6.5 Tumor Immunity immune/inflammatory cells, organ-specific chemo-attractants, growth factors, and ECM-­ Apart from its antiangiogenic potential, TSP-1 modifying proteins provide permissive microen- has the ability to reprogram immune microenvivironments that support extravasation, ronment. Various immune and myeloid compartcolonization, and metastatic outgrowth of DTCs ments are affected by TSP-1. TSP-1 mediates complex effects on macro[136, 137]. The PN has become an exciting area of research in the quest for novel therapeutic and phages. TSP-1 via α6β1 enhances M1 macroprophylactic strategies against metastasis [138, phage differentiation and recruitment at tumor sites, promoting tumor cell death via superoxide 139]. Primary tumor-derived TSP-1 has been shown production [39]. TSP-1-CD47 interaction inhibto reduce the growth of spontaneous lung metas- its IL-12 release by macrophages [145]. TSP-1 tasis in melanoma patients and murine melanoma also inhibits LPS-induced IL-1β transcription in models [140]. In the absence of this tumor-­ macrophages by limiting CD47-CD14 interacderived TSP-1, lung metastasis had higher levels tion [146]. Through CD36 receptor, TSP-1 mediof neovascularization and tumor growth at sec- ates IL-10 production in macrophages [147] and ondary sites. In contrast, a novel mechanism was partially mediates the activation of TLR-4 pathrecently described, whereby metastasis-­way in macrophages [38]. In cardiovascular incompetent tumors generate metastasis-­physiology, TSP-1-CD47 interaction and suppressive microenvironments in the lungs by NADPH oxidase 1 signaling are critical for the inducing the expression of TSP-1, in the recruited macropinocytic uptake of native LDL by macrobone marrow-derived myeloid cells [141]. TSP-1 phages [148]. Endothelial cells also increase induction was mediated by the activity of prosa- TSP-1 levels via B-raf/MEK/ERK pathway to posin (PSAP), a protein secreted by poorly meta- allow the recruitment of macrophages at these static cells, which acts systemically to reprogram sites and enable clearance of apoptotic cells [40]. myeloid cells into metastasis-inhibitory cells TSP-1 synthesized by monocytes is also known [141]. As another mechanism, inflammation in to play a role in the killing of undifferentiated lungs led to the recruitment of bone marrow-­ squamous carcinoma cells [98]. TSP-1 null mice derived neutrophils, which through the process of show symptoms of acute pneumonia with accudegranulation released Ser proteases: cathepsin mulation of neutrophils and macrophages in the G and elastase. These enzymes lead to the proteo- lungs [149]. TSP-1 may serve as an important lytic degradation of TSP-1, causing metastatic target to control obesity-induced inflammation, outgrowth [142]. as TSP-1-deficient mice show reduced macro-

8  Thrombospondin in Tumor Microenvironment

phage accumulation and inflammatory cytokine production than wild-type mice [150]. In dendritic cells (DCs), TSP-1 acts as an autocrine negative regulator [43]. Human iDCs produce TSP-1 which is enhanced by PGE2 and TGF-β secreted by phagocytic macrophages. This TSP-1 interacts with CD47 to reduce the secretion of IL-12, TNF-α, and IL-10, leading to a reduction in protective and inflammatory immune responses [43]. TSP-1 also is responsible for dysfunctional CD1a + MDDC (a DC subset differentiated from circulating human peripheral blood MO in response to specific inflammatory cytokines/pathogens) differentiation via activation of the inhibitory phosphatase, SHP-1 through CD47 receptor, leading to reduced T-cell stimulation and defective inflammatory immune responses [44]. CD47 is also critical for proper DC trafficking to elicit an immune response [42]. In support of the negative impact of TSP-1 on DC function, it was demonstrated that loss of TSP-1, but not TSP-2  in DCs, increased anti-tumor immune responses by increasing infiltration of CD4+ and CD8+ T lymphocytes and enhanced the production of IL-12 and IFN-γ, resulting in delayed tumor growth [45]. TSP-1 also mediates a negative impact on NK cells. TSP inhibits early NK cell proliferation but stimulates late NK cell expansion in a TGFβ-­dependent fashion [41]. TSP-1 has been implicated in immunosuppressive responses via induction of Tregs [151] and possibly TGF-β [152]. In a melanoma model of EMT, Snail promotes immunosuppressive effects by inducing Tregs and impairing dendritic cells, expressing high levels of IDO via TSP-1 production. Inhibition of either Snail or TSP-1 increased systemic immune responses, associated with decreased tumor growth and metastasis [37]. These immune responses were found to be independent of TGF-β. However, ­TGF-β2-­ expressing antigen-presenting cells require TSP-1 in the microenvironment in order to induce FOXp3+ Tregs both in vitro and in vivo [153]. TSP-1 is an endogenous inhibitor of T-cell activation and differentiation via its interaction with CD47 receptor. TSP-1 inhibits hydrogen

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sulfide-mediated MEK/ERK signaling in T cells for their activation. TSP-1 also inhibits autocrine activation of T cells by hydrogen sulfide via downregulation of cystathionine β-synthase and cystathionine γ-lyase enzymes (biosynthetic H2S enzymes) in T cells [35].

8.7

Future Commentary

Thrombospondins affect cellular functions by autocrine, paracrine, and indirect mechanisms that may involve interaction with proteins like TGF-β and lead to tissue remodeling, endothelial cell behavior, and cancer suppression/progression. Secreted by both tumor cells and the stroma, TSPs can mediate both suppressive and cancer-­promoting behaviors. Cancer-promoting roles may warrant inhibition of TSPs as a therapeutic modality to suppress tumor growth/progression. Therapeutic strategies targeting TSP-1 are already being pursued which include either TSP-­mimetic peptides, antibody blockades, or strategies to upregulate endogenous TSPs. Compounds like ABT-510 (TSR1s) and CVX045 (TSP-1 mimetic) have been in Phase II and Phase I clinical trials, respectively [17]. ABT510 was discontinued from clinical trials due to its low objective response rate and 1-hour halflife [17]. Although CVX-045 demonstrated efficacy in combination with chemotherapy, severe adverse events (radiation pneumonitis and bowel obstruction with perforation leading to death) associated with CVX-045 led to discontinuation of its further development [154]. ABT-898, a second-­generation TSP-1 mimetic peptide with increased stability and reduced toxicities, has been indicated for inhibiting ovarian cancer progression. Trabectedin, indicated for advanced soft tissue sarcoma and ovarian cancer, acts via upregulating TSP-1. An increase in TSP-1 via prosaposin (psap) provides a suitable translational potential as an anti-metastatic therapy. Psap, a five-amino acid peptide, which is known to stimulate stromal p53 and TSP-1, has been shown to be a potent anti-metastatic agent in various cancer models [113, 141, 155]. Administration of psap causes production and

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release of TSP-1 by monocytes present in the pre-metastatic niche, which then prevents metastatic colonization [113]. With multiple receptors binding to TSPs, there are numerous signaling pathways activated downstream which regulate complex dynamic processes in both normal physiology and pathology of a disease. Understanding these intercalating networks that act through TSPs is essential in developing the appropriate therapeutic strategy, which could have impact on the tumor microenvironment to suppress tumor progression using one of the cellular activities (angiogenesis, tumor immunity, etc.) affected by thrombospondins.

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9

Tenascin-C Function in Glioma: Immunomodulation and Beyond Fatih Yalcin, Omar Dzaye, and Shuli Xia

F. Yalcin · O. Dzaye Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD, USA

of toll-like receptor 4 and promotes inflammatory response by inducing the expression of multiple pro-inflammatory factors in innate immune cells such as microglia and macrophages. In addition, TNC drives macrophage differentiation and polarization predominantly towards an M1-like phenotype. In contrast, TNC shows immunosuppressive function in T cells. In glioma, TNC is expressed by tumor cells and stromal cells; high expression of TNC is correlated with tumor progression and poor prognosis. Besides promoting glioma invasion and angiogenesis, TNC has been found to affect the morphology and function of tumor-associated microglia/macrophages in glioma. Clinically, TNC can serve as a biomarker for tumor progression; and TNC antibodies have been utilized as an adjuvant agent to deliver anti-tumor drugs to target glioma. A better mechanistic understanding of how TNC impacts innate and adaptive immunity during tumorigenesis and tumor progression will open new therapeutic avenues to treat brain tumors and other malignancies.

Department of Radiology and Neuroradiology, Charité, Berlin, Germany

Keywords

Abstract

First identified in the 1980s, tenascin-C (TNC) is a multi-domain extracellular matrix glycoprotein abundantly expressed during the development of multicellular organisms. TNC level is undetectable in most adult tissues but rapidly and transiently induced by a handful of pro-inflammatory cytokines in a variety of pathological conditions including infection, inflammation, fibrosis, and wound healing. Persistent TNC expression is associated with chronic inflammation and many malignancies, including glioma. By interacting with its receptor integrin and a myriad of other binding partners, TNC elicits context- and cell type-dependent function to regulate cell adhesion, migration, proliferation, and angiogenesis. TNC operates as an endogenous activator

S. Xia (*) Hugo W. Moser Research Institute at Kennedy Krieger, Baltimore, MD, USA Department of Neurology, Johns Hopkins School of Medicine, Baltimore, MD, USA e-mail: [email protected]

Tenascin-C · Extracellular matrix · Brain · Glioma · Tumor microenvironment · Integrin · Toll-like receptor · Adhesion · Proliferation · Angiogenesis · Cancer stem cells

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 A. Birbrair (ed.), Tumor Microenvironment, Advances in Experimental Medicine and Biology 1272, https://doi.org/10.1007/978-3-030-48457-6_9

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· Immunomodulation · Inflammation · Tumor-­associated microglia/macrophages · T cells

Abbreviations CNS Central nervous system ECM Extracellular matrix EGF Epidermal growth factor EMT Epithelial-mesenchymal transition FBG Fibrinogen-like globe FGF Fibroblast growth factor FNIII Fibronectin type III-like domains GBM Glioblastoma GSCs GBM cancer stem cells HA Hyaluronic acid IL Interleukin LPS Lipopolysaccharide MMP Matrix metalloproteinases PDGF Platelet-derived growth factor PRR Pattern recognition receptors TAMs Tumor-associated microglia/ macrophages TGF-β Transforming growth factor-beta TLR Toll-like receptor TME Tumor microenvironment TNC Tenascin-C TNFα Tumor necrosis factor-α

9.1

Introduction

Tenascin-C (TNC) is a multifunctional extracellular matrix (ECM) glycoprotein with distinct spatial and temporal expression patterns during embryonic development, tissue homeostasis, and disease, including various forms of solid malignancies. Over the last several decades, many reports have shown how TNC modulates cell adhesion, migration, proliferation, and angiogenesis in a context- and cell type-specific manner. However, little is known about its immunomodulatory function in pathologies, ­

including malignant tumors of the central nervous system (CNS). In the following chapter, we are going to briefly discuss TNC structure, receptors/interaction partners, expression, and regulation in general, with an emphasis on brain tumors. We will focus on TNC function in cell-specific immunomodulation and discuss current therapeutic interventions as well as future directions.

9.2

Structure of Tenascin-C

TNC is the first member of the tenascin family discovered by various groups in the 1980s [34]. Other members of the family, tenascin-W, tenascin-­X, tenascin-R, and tenascin-Y, show structural similarity with TNC and overlap with TNC function during development, wound healing, tissue remodeling, and diseases [94]. Since most discoveries and functional studies have been related to TNC, in this chapter, we will only focus on TNC. The TNC protein consists of six subunits linked by bisulfide bonds; each subunit is ~250  kDa and has four distinct domains: a cysteine-­rich amino terminus with heptad repeats, 14.5 epidermal growth factor (EGF)-like repeats, eight constitutive fibronectin type III-like (FNIII) domains, and a C-terminal fibrinogen-like globe (FBG) [94]. These motifs generate a highly versatile structure, due to which TNC has the capacity to interact with multiple proteins [131]. For an illustration of TNC domains, please refer to many other reviews [94, 155]. Contributing to the multi-modular structure of TNC is its expression in several variants [35], which are due to alternative splicing and post-­ translational modifications, such as glycosylation. Western blotting analysis of the TNC protein from human brain tumor cells showed multiple bands between 250 and 300 kDa [204]. In human TNC, there are eight FNIII repeats (FNIII 1–8) constitutively presented. Between the fifth and sixth FNIII domain, a series of FNIII domains (up to nine) may be presented, due to alternative splicing in a varying fashion [131]. These highly heterogeneous FNIII domains of TNC can generate more than a cou-

9  Tenascin-C Function in Glioma: Immunomodulation and Beyond

ple of hundred potential isoforms that differ in proteolytic cleavage sites and post-translational modifications [59].

9.3

Tenascin-C Receptors and Interaction Partners

TNC’s highly versatile structure allows it to interact with various ligands, for example, the ECM itself [39], integrins [214], pattern recognition receptors (PRR) [226], and soluble factors [44]. The most well-studied ECM binding partner of TNC is fibronectin, which interacts with TNC via the binding sites located throughout the FNIII repeats of TNC [39]. Other ECM-related molecules that bind to TNC are collagens [135], periostin [104], SMOC1 [18], fibrillin-2 [19], and proteoglycans of the lectin family [7]. The capability to bind ECM components suggests that TNC might play a role in the structural organization of the tumor microenvironment; evidence of such mechanisms mediated by TNC already exists in brain tumor [20]. In addition, TNC has been shown to affect ECM stiffness and mechanosignaling in various tissues [88]. TNC also binds to syndecan-4 [173], glypican [2], and receptor-like protein tyrosine phosphatase beta zeta (RPTPβ/ζ) [4], among a few other membrane-­ binding proteins. For a detailed description of TNC binding sites to these partners, please see Midwood et al. [132]. The interactions between integrins and the ECM molecules play an important role in cancer biology by propagating metastasis and invasion [68]. Seven integrins have already been shown to directly bind to TNC FNIII and FBG domains, including α9β1, αvβ3, α8β1, αvβ6, α2β1, a7β1, α7β1, and α8β1 [196]. The general function of these TNC-integrin interactions in tumors is associated with epithelial-mesenchymal transition (EMT), migration, and poor patient survival [214]. TNC takes part in innate immunity by directly binding to toll-like receptor 4 (TLR4) on immune cells [130] or pathogens, e.g., the human immunodeficiency virus (HIV) [54]. The EGF-like repeats of TNC have been shown to bind directly

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to the EGF receptor (EGFR) and promote cell proliferation [191]. A growing number of TNC interaction partners are soluble factors [44]. These consist of a wide spectrum of growth factors, which include members in the platelet-derived growth factor (PDGF) family, the fibroblast growth factor (FGF) family, the transforming growth factor-­ beta (TGF-β) superfamily, the insulin-like growth factor binding proteins, and neurotrophins. All these interactions are mediated by TNC FNIII 4–5 subdomain [44, 126]. TNC has also been described to promote Wnt/β-catenin signaling by binding to Wnt3a in the whisker follicle stem cell niches [78] and injured kidney [28].

9.4

Expression of Tenascin-C in Glioma

Unlike the periphery system, in which the main ECM components are fibrillar collagen, fibronectin, laminin, etc., the ECM of the CNS is enriched with various non-fibrillar components such as proteoglycans and glycoproteins [163]. Together with thrombospondin-1 (TSP-1) and SPARC, TNC belongs to the matricellular proteins, which are secreted and rapidly turned over molecules in the ECM with mainly regulatory functions instead of structural supportive roles [16]. In contrast to other ECM components in the CNS, e.g., the abundant expressed hyaluronic acid (HA), TNC is only highly expressed during normal fetal development [137]. In support of TNC expression in the developing brain, persistent levels of TNC have been characterized in adult neural stem cell niches, including the radial glia and astrocyte stem cell compartment of the subventricular zone (SVZ) [50]. Other parts of adult brains have an undetectable level of TNC, and persistent TNC expression is associated with a variety of chronic neuropathological conditions, including brain cancer. Grade IV glioma, glioblastoma (GBM), is the most common and malignant primary brain tumor in adults and comprises ~25,000 new cases annually in the United States [182]. The median survival of patients diagnosed with GBM is less

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than 2 years. The ECM in the glioma microenvironment is often deregulated [197], and abnormalities in the ECM affect cancer progression by directly promoting cellular transformation, tumor cell migration, invasion, tumor-associated angiogenesis, and tumor-associated inflammation [51]. TNC is abundantly expressed in GBM [25] as well as multiple other solid malignancies, including breast cancer [17], prostatic adenocarcinoma [102], and colorectal carcinoma [73]. In solid tumors of the periphery system, fibroblasts in the tumor stroma are a major source of TNC deposited in the tumor microenvironment (TME) [70, 82]. In contrast, in GBM, malignant tumor cells are the main source of TNC production [202]. Besides neoplastic cells, endothelial cells have also been found to be a source of TNC in the CNS [21, 125]. Complimentary studies establish the prognostic value of TNC as a biomarker for poor patient survival and disease progression [41, 61, 114, 148, 162, 210–212]. In glioma, TNC has been established to correlate with its respective grade [105]. In TCGA, data of low-grade glioma (LGG) and high-grade glioma (HGG) display a significant difference in survival between patients with TNC-high (median  =  25.2  months) and TNC-low (median  =  95.6  months) expression (Fig. 9.1). Many solid tumors, including GBM, exhibit cellular heterogeneity and differentiation hierar-

chy in which GBM cancer stem cells (GSCs) are at the apex [186]. GSCs exhibit the capacity of self-renewal, multi-lineage differentiation, and tumor initiation [56, 213]. Multiple studies, including mass spectrometry and lectin-­ microarrays [76, 145], established TNC as a specific cell surface marker for GSCs. In Xia et al., immunofluorescence staining of TNC expression in orthotopic xenografts derived from GSCs showed extensive and strong TNC expression in the extracellular space of brain tumors [204]. The perivascular niche is a well-established compartment of the tumor microenvironment, known for its enrichment of GSCs [23]. Broad expression analysis of TNC in human gliomas of varying grades revealed the association of TNC in this particular niche, and TNC level in the perivascular niche correlated with angiogenesis and a shorter disease-free time [79, 217]. As mentioned earlier, depending on its transcriptional and post-transcriptional modifications, TNC appears in a variety of isoforms [35]. Characterization of specific TNC isoforms in the tumor microenvironment has shown that the large isoform of TNC is abundantly expressed in multiple solid malignancies, including GBM [17, 25, 73, 102]. Monoclonal antibodies (mAb) designed to interact with specific epitopes of TNC large isoforms displayed three distinct staining patterns in human glioma [21]: the mAbs stained

Fig. 9.1 (a) Expression of TNC in non-tumor tissues (n = 10) and grade II (n = 226), III (n = 244), and IV gliomas (n = 150). (b) Kaplan-Meier survival curve in TNChigh

(n  =  334) versus TNClow (n  =  336) patients. Cutoff, median; ∗∗∗∗, p-value  =